Add from_fork scope

What does this MR do?

In !22418 (merged), a change has been made that required the need to disable the CodeReuse/ActiveRecord cop on merge_requests_for_forks.

To be able to enable it again, added from_fork scope and use that instead.

Does this MR meet the acceptance criteria?

Conformity

• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)

Related to #201745 (closed)