Ensure MergeRequestsFinder uses correct Params

What does this MR do?

Ruby does some weird things when dynamically loading classes. It seems in some cases it did not load MergeRequestsFinder::Params when it's calling #params_class. In that case the superclass method is called instead.

We discovered this issue in IssuesFinder before, and it was addressed in !41835 (merged). This change applies the same change to MergeRequestsFinder.

Closes #296939 (closed)

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