dhcpv6-server: T5992: Fix op-mode Kea DHCP lease output, migration from deprectaed utcfromtimestamp to fromtimestamp

[nvandamme]

Change summary

Fix datetime handling for leases show (there were some error due to missing utc tz, thus using a "naive" timezone representation from timestamp) and migrate from deprecated uctfromtimestamp to datetime function with enforced utc timezone.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related PR(s)

How to test / Smoketest result

show dhcpv6 server leases

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• [] I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

❌
PR title does not match the required format

[nvandamme]

Already corrected by fe50656

[github-actions]

CI integration ❌ failed!

Details

CI logs

• CLI Smoketests (no interfaces) ❌ failed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed