Ensure merge request's "remove branch" accessors return booleans
What does this MR do?
- Ensures it's possible to disable branch removal when editing a merge request
- Ensures
MergeRequest#should_remove_source_branch?andMergeRequest#force_remove_source_branch?return booleans (even when'1'or'0'is stored in the serialized DB column.
Fixes #24102 (closed)