Respect the timezone reported from Gitaly for commits

What does this MR do?

Git commit timestamps (both author and committer) consist of two values: a timestamp, and a timezone. gitaly!1594 (merged) added support for relaying the timezone when reading information about commits, but right now, gitlab itself ignores that information.

This MR reads the timezone field and uses that, instead of UTC, in conjunction with the timestamp. This means we can correctly display the time for a commit when it's non-UTC.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #37385