Remove job_age from jobs/request API endpoint

What does this MR do and why?

This feature was likely never working as intended because of caching, and the parameter does not seem to be in use on GitLab.com.

The removal is behind a feature flag that is enabled by default, but will be explicitly disabled on GitLab.com prior to merging this MR. That way, if we by any chance miss the feature flag cleanup in 16.0, the feature will still be removed.

This removal was announced in 15.2, see !91509 (merged)

See #334253 (closed)

Pre-merge checklist

Feature has been explicitly disabled in #production, see slack (internal link).

To verify, please check #production:

/chatops run feature get remove_job_age_from_jobs_api

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.