Backport of 'Send canonical timezone identifiers to Gitaly'

What does this MR do and why?

Backport of !257897 (merged) to 19-4-stable-ee.

On GitLab 19.4 Cloud Native (CNG) installs, merge requests showed false merge conflicts when the author's stored timezone was a legacy IANA link name. GitLab sends the user's timezone to Gitaly, which resolves it with Go's time.LoadLocation. CNG 19.4 moved to Debian trixie, which ships legacy timezone links only in the tzdata-legacy package, and the Gitaly image did not install it, so Gitaly failed with unknown time zone.

This change adds Gitlab::Git::LegacyTimezones::MAPPING, a table mapping the legacy links Debian ships only in tzdata-legacy to their canonical zones, and translates the resolved identifier (and the instance fallback) through it before sending it to Gitaly. Stored user settings are unchanged. No data migration, no UI change.

This is a clean cherry-pick of the squashed commit from the default branch MR. The four changed files are byte-identical to the merged version.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.