Add docs for mysql caching_sha2_password default

[huwd]

This stumped me for a couple of days.

Since the mysql 8.0 update I had been unable to make whitehall seeing errors when i tried to run: govuk-docker run whitehall-lite bundle exec rails db:prepare.

The stack trace was as follows:

ActiveRecord::ConnectionNotEstablished: Plugin caching_sha2_password could not be loaded: /usr/lib/x86_64-linux-gnu/mariadb19/plugin/caching_sha2_password.so: cannot open shared object file: No such file or dire
ctory                                                                                                                                                                                                              
/govuk/whitehall/app/models/edition/translatable.rb:19:in `block in <module:Translatable>'                                                                                                                         
/govuk/whitehall/app/models/edition.rb:23:in `include'                                                                                                                                                             
/govuk/whitehall/app/models/edition.rb:23:in `<class:Edition>'                                                                                                                                                     
/govuk/whitehall/app/models/edition.rb:3:in `<main>'                                                                                                                                                               
/govuk/whitehall/app/models/publicationesque.rb:9:in `<main>'                                                                                                                                                      
/govuk/whitehall/app/models/publication.rb:8:in `<main>'                                                                                                                                                           
/govuk/whitehall/app/helpers/admin/edition_routes_helper.rb:4:in `<module:EditionRoutesHelper>'                                                                                                                    
/govuk/whitehall/app/helpers/admin/edition_routes_helper.rb:1:in `<main>'                                                                                                                                          
/govuk/whitehall/config/initializers/teaspoon.rb:2:in `<main>'                                                                                                                                                     
/govuk/whitehall/config/environment.rb:5:in `<main>'                                                                                                                                                               
                                                                                                                                                                                                                   
Caused by:                                                                                                                                                                                                         
Mysql2::Error::ConnectionError: Plugin caching_sha2_password could not be loaded: /usr/lib/x86_64-linux-gnu/mariadb19/plugin/caching_sha2_password.so: cannot open shared object file: No such file or directory   
/govuk/whitehall/app/models/edition/translatable.rb:19:in `block in <module:Translatable>'                                                                                                                         
/govuk/whitehall/app/models/edition.rb:23:in `include'                                                                                                                                                             
/govuk/whitehall/app/models/edition.rb:23:in `<class:Edition>'                                                                                                                                                     
/govuk/whitehall/app/models/edition.rb:3:in `<main>'                                                                                                                                                               
/govuk/whitehall/app/models/publicationesque.rb:9:in `<main>'                                                                                                                                                      
/govuk/whitehall/app/models/publication.rb:8:in `<main>'                                                                                                                                                           
/govuk/whitehall/app/helpers/admin/edition_routes_helper.rb:4:in `<module:EditionRoutesHelper>'                                                                                                                    
/govuk/whitehall/app/helpers/admin/edition_routes_helper.rb:1:in `<main>'                                                                                                                                          
/govuk/whitehall/config/initializers/teaspoon.rb:2:in `<main>'                                                                                                                                                     
/govuk/whitehall/config/environment.rb:5:in `<main>'                                                                                                                                                               
Tasks: TOP => db:prepare => db:load_config => environment                                                                                                                                                          
(See full trace by running task with --trace)

This seems to be related to a change in MySQL password default config in v8.
This documents a solution, (with massive thanks to @ChrisBAshton and @brucebolt)

We may want to find a more sustainable solution as part of the MySQL docker setup.
I have created an issue here for that.

[alex9smith]

LGTM, one non-blocking request

[ChrisBAshton]

Thanks a lot for getting this documented!

I'm not sure it needs quite such a detailed introduction (perhaps some of the context can be added to the commit message itself, rather than the doc), as most devs will just want to dive in and copy and paste the fixes. But happy to leave it in if it's useful 👍

[huwd]

Great thanks both, will get those amended today

[ChrisBAshton]

LGTM 👍 one non-blocking comment if you fancy addressing before merging. Shouldn't need a re-review from me.