Rebuild trigram indexes.

What does this MR do?

This MR rebuilds all trigram indexes we have.

Migration process (upwards) can be viewed here: https://gitlab.com/gitlab-org/gitlab-ce/snippets/1721534 Down: https://gitlab.com/gitlab-org/gitlab-ce/snippets/1721536 (although there's nothing to clean up obviously)

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

Consistency, especially when aborting the migration in the middle of it (or what happens if it fails).

Why was this MR needed?

See gitlab-com/database#14 (moved).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

gitlab-com/database#14 (moved)