Remove unused index_oauth_access_grants_on_created_at_expires_in

What does this MR do and why?

Remove unused index_oauth_access_grants_on_created_at_expires_in

Remove the unused index_oauth_access_grants_on_created_at_expires_in index on oauth_access_grants that was previously added to support the cleanup worker but is no longer being used.

Related to: #562376

Metrics showing the unused index:

• Grafana pg_stat_user_indexes_idx_scan
• Grafana sum by

References

• Issue: #562376
• Related worker: !207837 (merged)
• https://docs.gitlab.com/development/database/adding_database_indexes/#dropping-unused-indexes

Database

Migration up

>>> Executing: bin/rails db:migrate:up:main VERSION=20251205113108
DEPRECATION WARNING: `config.active_record.warn_on_records_fetched_greater_than` is deprecated and will be removed in Rails 8.0. Please subscribe to `sql.active_record` notifications and access the row count field to detect large result set sizes. (called from <main> at /Users/daniele/projects/gdk/gitlab/config/environment.rb:7)
main: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 76111
main: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: migrating =
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0193s
main: -- indexes(:oauth_access_grants)
main:    -> 0.0028s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:oauth_access_grants, {:algorithm=>:concurrently, :name=>"index_oauth_access_grants_on_created_at_expires_in"})
main:    -> 0.0025s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: migrated (0.0946s) 

main: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 76111
>>> Executing: bin/rails db:migrate:up:ci VERSION=20251205113108
DEPRECATION WARNING: `config.active_record.warn_on_records_fetched_greater_than` is deprecated and will be removed in Rails 8.0. Please subscribe to `sql.active_record` notifications and access the row count field to detect large result set sizes. (called from <main> at /Users/daniele/projects/gdk/gitlab/config/environment.rb:7)
ci: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 76195
ci: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: migrating =
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0169s
ci: -- indexes(:oauth_access_grants)
ci:    -> 0.0035s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- remove_index(:oauth_access_grants, {:algorithm=>:concurrently, :name=>"index_oauth_access_grants_on_created_at_expires_in"})
ci:    -> 0.0018s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: migrated (0.0979s) 

ci: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 76195

Migration down

>>> Executing: bin/rails db:migrate:down:main VERSION=20251205113108
DEPRECATION WARNING: `config.active_record.warn_on_records_fetched_greater_than` is deprecated and will be removed in Rails 8.0. Please subscribe to `sql.active_record` notifications and access the row count field to detect large result set sizes. (called from <main> at /Users/daniele/projects/gdk/gitlab/config/environment.rb:7)
main: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 75309
main: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: reverting =
main: -- transaction_open?(nil)
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0202s
main: -- index_exists?(:oauth_access_grants, [:created_at, :expires_in], {:name=>"index_oauth_access_grants_on_created_at_expires_in", :algorithm=>:concurrently})
main:    -> 0.0028s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- add_index(:oauth_access_grants, [:created_at, :expires_in], {:name=>"index_oauth_access_grants_on_created_at_expires_in", :algorithm=>:concurrently})
main:    -> 0.0038s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: reverted (0.0985s) 

main: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 75309
>>> Executing: bin/rails db:migrate:down:ci VERSION=20251205113108
DEPRECATION WARNING: `config.active_record.warn_on_records_fetched_greater_than` is deprecated and will be removed in Rails 8.0. Please subscribe to `sql.active_record` notifications and access the row count field to detect large result set sizes. (called from <main> at /Users/daniele/projects/gdk/gitlab/config/environment.rb:7)
ci: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 75397
ci: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: reverting =
ci: -- transaction_open?(nil)
ci:    -> 0.0000s
ci: -- view_exists?(:postgres_partitions)
ci:    -> 0.0178s
ci: -- index_exists?(:oauth_access_grants, [:created_at, :expires_in], {:name=>"index_oauth_access_grants_on_created_at_expires_in", :algorithm=>:concurrently})
ci:    -> 0.0031s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- add_index(:oauth_access_grants, [:created_at, :expires_in], {:name=>"index_oauth_access_grants_on_created_at_expires_in", :algorithm=>:concurrently})
ci:    -> 0.0021s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20251205113108 RemoveIndexOauthAccessGrantsOnCreatedAtExpiresIn: reverted (0.0975s) 

ci: == [advisory_lock_connection] object_id: 130900, pg_backend_pid: 75397

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