Add indices for sharding_key_id for runners and runner managers

What does this MR do and why?

This MR adds indices for sharding_key_id on ci_runners and ci_runner_machines. This will allow for efficient lookups when a project is deleted and we need to find all runners and runner managers that point to that project. See issue for more context.

I've followed the process described in https://docs.gitlab.com/ee/development/database/adding_database_indexes.html#add-an-index-to-support-new-or-updated-queries, and plan to merge a follow-up MR that will leverage these indices in response to projects being deleted, only when this MR is deployed to production on .com.

Changelog: added

References

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• Update sharding_key_id for runners tables shard... (#511343 - closed)

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