WIP: Resolve "Should project updated time actually be the last activity time?"

What does this MR do?

• Update Project list sort option from using updated_at to using last_activity_at
• Sorts Project list according to the last activity on that project (including issues, merge requests, pushed code)

Why was this MR needed?

• Projects list is current sorted on when the Project object was last updated, which isn't what would naturally be expected

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
  • 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 #26510 (closed)