Use update_columns to by_pass all the dirty code on active_record

What does this MR do?

Speed up storing current content of merge_request_diff instances, avoiding YAML deserialization and serialization

What are the relevant issue numbers?

#18663 (moved) one of the points of improvement

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together