Always regenerate MR cached HTML for selected projects

What does this MR do and why?

Always regenerate MR cached HTML for selected projects

Add a project-scoped, disabled-by-default feature flag (regenerate_merge_request_cached_html) that forces a merge request's title_html/description_html to be treated as stale so they are regenerated and re-persisted on read/write. This lets us observe the cold-cache regeneration path continuously for a selected project.

Staleness is only forced until the first regeneration on an instance to avoid recursing through store_mentions; a freshly loaded record (a new request) regenerates again.

References

Issue - https://gitlab.com/gitlab-org/gitlab/-/work_items/602026+

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