Clear merge requests counter cache after merge

What does this MR do?

Refresh the MR counter cache after merging an MR.

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

Don't think so.

Why was this MR needed?

We did this on every MR state change event except a merge, so if the last change to an MR's state was a merge, the counter would be wrong.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/3573 and https://gitlab.com/gitlab-org/gitlab-ce/issues/38344.