What does this MR do and why?

Allows users to clean up stale environments that haven't been modified or deployed to since a specified date

Screenshots or screen recordings

How to set up and validate locally

1. Enable the stop_stale_environments feature flag (i.e. Feature.enable(:stop_stale_environments))
2. Navigate to the environments page for a project that has stale environments as a user with maintainer access
3. Click "Clean up environments" to show the new modal
4. Input a date after the stale environments you want to stop have been updated
5. Click "clean up"
6. Wait for stale environments to become "stopped" (should be fairly quick)

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.

Related to #19724 (closed)