Resolve "Number of required approvals MUST NOT be reset when changing assignee"

What does this MR do?

Don't reset the approval count set on an MR when it's higher than the project default, and we're not passing an approvals_before_merge param.

Are there points in the code the reviewer needs to double check?

Don't think so.

Why was this MR needed?

It was a bug! If the param isn't passed, we don't need to do the whole dance with clamping it.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #1961 (closed).