Merge branch '23234-deleting-a-milestone-removes-source-branch-deletion-options-of-associated-merge-requests' into 'master'

Maintain "force_remove_source_branch" options on Merge Request unless specified

## What does this MR do?

Fixes a problem where Merge Requests were losing the state associated with the flag to remove the source branch upon merge

* Closes #23234 
* Closes #23191
* Closes #19351

See merge request !6817