Add confidential notes index

What does this MR do and why?

In a previous step (!89520 (merged)) this index was prepared on .com and verified that this index exists there:

https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/10544/commands/37865

\d

Query:

index_notes_on_id_where_confidential

Response:

Index "public.index_notes_on_id_where_confidential"
 Column |  Type   | Key? | Definition 
--------+---------+------+------------
 id     | integer | yes  | id
btree, for table "public.notes", predicate (confidential = true)

Currently we create this index synchronously also for self-managed instances (as described in https://docs.gitlab.com/ee/development/adding_database_indexes.html#add-a-migration-to-create-the-index-synchronously)

Screenshots or screen recordings

Migration output

honza@pc ~/gitlab-development-kit/gitlab (jp-sync_confidential_idx)$ rake db:migrate
main: == 20220613095911 CreateConfidentialNotesIndexOnId: migrating =================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:notes)
main:    -> 0.0081s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:notes, {:algorithm=>:concurrently, :name=>"index_notes_on_confidential"})
main:    -> 0.0053s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:notes, :id, {:where=>"confidential = true", :name=>"index_notes_on_id_where_confidential", :algorithm=>:concurrently})
main:    -> 0.0056s
main: -- add_index(:notes, :id, {:where=>"confidential = true", :name=>"index_notes_on_id_where_confidential", :algorithm=>:concurrently})
main:    -> 0.0079s
main: == 20220613095911 CreateConfidentialNotesIndexOnId: migrated (0.0346s) ========

ci: == 20220613095911 CreateConfidentialNotesIndexOnId: migrating =================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:notes)
ci:    -> 0.0075s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- remove_index(:notes, {:algorithm=>:concurrently, :name=>"index_notes_on_confidential"})
ci:    -> 0.0044s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- index_exists?(:notes, :id, {:where=>"confidential = true", :name=>"index_notes_on_id_where_confidential", :algorithm=>:concurrently})
ci:    -> 0.0057s
ci: -- Index not created because it already exists (this may be due to an aborted migration or similar): table_name: notes, column_name: id
ci: == 20220613095911 CreateConfidentialNotesIndexOnId: migrated (0.0209s) ========

honza@pc ~/gitlab-development-kit/gitlab (jp-sync_confidential_idx)$ rake db:rollback:ci db:rollback:main
ci: == 20220613095911 CreateConfidentialNotesIndexOnId: reverting =================
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:notes)
ci:    -> 0.0081s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0003s
ci: -- remove_index(:notes, {:algorithm=>:concurrently, :name=>"index_notes_on_id_where_confidential"})
ci:    -> 0.0198s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0003s
ci: == 20220613095911 CreateConfidentialNotesIndexOnId: reverted (0.0335s) ========

main: == 20220613095911 CreateConfidentialNotesIndexOnId: reverting =================
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:notes)
main:    -> 0.0072s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- remove_index(:notes, {:algorithm=>:concurrently, :name=>"index_notes_on_id_where_confidential"})
main:    -> 0.0041s
main: -- execute("RESET statement_timeout")
main:    -> 0.0002s
main: == 20220613095911 CreateConfidentialNotesIndexOnId: reverted (0.0137s) ========

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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