Replace token indices in ci_runners table with unique versions

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR implements a few post-deployment migrations to replace the existing ci_runners indices on token and token_encrypted with unique versions of those indices. We already ensured that no duplicates exist in the ci_runners table in Nullify duplicate runner authentication tokens (!98689 - merged), and call finalize_background_migration to ensure that those migrations have finished running before adding the unique indices. This MR is very similar to Replace runners token indices in projects table (!90501 - merged) from %15.2 which performed the same logic for runner registration tokens in the projects table.

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