MR API: Allow `allow_{collaboration,maintainer_to_push}` to be updated

What does this MR do?

It was reported in https://gitlab.com/gitlab-org/gitlab-triage/-/merge_requests/147#note_339431814 that it was impossible to update an MR with a single allow_collaboration or allow_maintainer_to_push attribute.

This MR fixes that.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done