Merge branch...

Merge branch '21636-deleting-source-project-with-existing-fork-closes-all-related-merge-requests' into 'master'

Deleting source project with existing fork link should closes all
related merge requests, and does not render `invalid` template for merge
requests with deleted source project.
The merge request without source project can be edited but can't be reopened.

Closes #21636 

See merge request !6177