Add `issues` and `merge_requests` to high traffic tables

What does this MR do?

Adds issues and merge_requests to the list of high-traffic tables that require the with_lock_retries helper.

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