Add index to oauth_access_grants.application_id

What does this MR do and why?

This MR adds an index to the application_id column to fix the timeout error when removing oauth applications. Internal issue: https://gitlab.com/gitlab-com/support/internal-requests/-/issues/18610

The table contains about 50m rows but it doesn't seem to be a high traffic table (not in the list).

MR acceptance checklist

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Related to #393247 (closed)