Add toggle to enable prune of stale runners

What does this MR do and why?

This change adds a toggle in the group settings to enable pruning of stale group runners. This toggle is behind a feature flag.

Related to #342605 (closed)

Screenshots or screen recordings

A new setting is added for GitLab Ultimate users:

Enabling the setting has a warning:

How to set up and validate locally

1. Ensure your group or instance have a GitLab Ultimate license.
2. Enable the feature flag stale_runner_cleanup_for_namespace_development.
3. Visit group settings for any group you own e.g. http://gdk.test:3000/groups/h5bp/-/settings/ci_cd
4. Select the Runners section.
5. Enable/disable the toggle to "Enable stale runner cleanup"
6. Confirm your selection with "Yes, start deleting stale runners"
7. Runners should be removed over time
   • See more in-depth instructions of how the runners are removed, see: !86477 (merged)

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.