Deleting a MR you are assigned to should decrements counter

What does this MR do?

it adds a call to User.update_cache_counts in the Issuable::DestroyService (if an assignee exists) so that the UI updates the summary of merge requests counts immediately (instead of waiting on the 20min cache expiration)

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

whether or not the cache busting can be done inline, or needs to be wrapped in a Service object (I saw that pattern in the code, but wasn't sure if it applied here)

Why was this MR needed?

To fix a bug report

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

• Closes #44458 (closed)