Replace runners token indices in projects table with unique versions

Having ensured that no duplicates remain in the projects table with !81991 (merged), we should now add a migration to create unique indices, drop the existing non-unique ones (index_projects_on_runners_token and index_projects_on_runners_token_encrypted), as well as the temporary indices created in !81991 (merged) (tmp_index_projects_on_id_and_runners_token and tmp_index_projects_on_id_and_runners_token_encrypted).

The following discussion from !81991 (merged) should be addressed:

• @ahegyi started a discussion:

  We cannot remove the index because the BG migration jobs depend on it. We need to create a follow-up issue for the index removal.