Rename default branch from master to main

[AlejandroFernandezLuces]

As title says.

Suggested in https://github.com/ansys-internal/pymechanical-meshworkflows/pull/51#discussion_r1905519900

This change is easy, but surely will break stuff in third repos. Maybe a migration plan should be defined?

[RobPasMue]

Definitely needs a migration plan. I think what was suggested works:

• Having master and main as two separate branches. Make main the default and sync back master during the migration period.
• Leave it as is for several months and work with the various libraries using this to adapt
• After migration on libraries is complete, completely deprecate master and freeze it.
• Eventually, deletion of master. I'm worried about backwards compatibility here for libraries that have already released versions using it... so this point is TBDetermined.