WIP: Drop legacy and unused ci_projects table

What does this MR do?

It drops the unused ci_projects table.

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

• Is downtime really needed when dropping an unused table?
• To the best of my understanding, there is no need to migrate any data since it has been done when that table was rendered obsolete but maybe there was a reason why this table wasn't dropped back then.

Why was this MR needed?

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 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 #23993 (closed)