Remove trigram indexes for "ci_runners"

What does this MR do?

This MR removes two trigram indexes from the ci_runners table.

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

No.

Why was this MR needed?

These indexes are only very rarely used while slowing down any update. See 4baee5d3 for more information.

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
• [x ] Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together