dhcp: T6998: Make dhcp lease datetime timezone aware

[indrajitr]

Change summary

This resurrects part of the fixes from PR #4277. They were approved but never rebased and applied.

In this PR:

• Cherry-picked bf2b73a
• Removed vestigial method _utc_to_local since it isn't used anywhere
• Renamed variables and reformatted

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 Task(s)

https://vyos.dev/T6998

Related PR(s)

#4277

How to test / Smoketest result

Before:

vyos@test-0122:~$ show dhcp server leases sort ip
IP Address    MAC address        State    Lease start          Lease expiration     Remaining                Pool    Hostname                     Origin
------------  -----------------  -------  -------------------  -------------------  -----------------------  ------  ---------------------------  --------
10.10.1.9     80:aa:bb:cc:dd:ee  active   2025-01-25 20:25:54  2025-01-26 20:25:54  13:48:03                 LAN1    ngap                         local
10.10.2.2     a6:aa:bb:cc:dd:ee  active   2025-01-25 11:23:18  2025-01-26 11:23:18  4:45:27                  LAN2    watch                        local
10.10.2.3     7a:aa:bb:cc:dd:ee  active   2025-01-25 19:56:42  2025-01-26 19:56:42  13:18:51                 LAN2    phone                        local

After:

vyos@test-0122:~$ show dhcp server leases sort ip
IP Address    MAC address        State    Lease start                Lease expiration           Remaining    Pool    Hostname                     Origin
------------  -----------------  -------  -------------------------  -------------------------  -----------  ------  ---------------------------  --------
10.10.1.9     80:aa:bb:cc:dd:ee  active   2025-01-26 08:25:54+00:00  2025-01-27 08:25:54+00:00  13:40:25     LAN1    ngap                         local
10.10.2.2     a6:35:47:2b:07:11  active   2025-01-25 23:23:18+00:00  2025-01-26 23:23:18+00:00  4:37:49      LAN2    watch                        local
10.10.2.3     7a:4a:66:d9:af:80  active   2025-01-26 07:56:42+00:00  2025-01-27 07:56:42+00:00  13:11:13     LAN2    phone                        local

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

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