Ensure merge request's "remove branch" accessors return booleans

What does this MR do?

1. Ensures it's possible to disable branch removal when editing a merge request
2. Ensures MergeRequest#should_remove_source_branch? and MergeRequest#force_remove_source_branch? return booleans (even when '1' or '0' is stored in the serialized DB column.

Fixes #24102 (closed)