Finalize background migration for discussion IDs

What does this MR do and why?

Finalizes the background migration added in 15.0: !78555 (merged)

Related to #346495 (closed), #357581 (closed)

We don't have any more notes with NULL discussion_id on staging and production:

gitlabhq_dblab=# SELECT COUNT(*) FROM notes WHERE discussion_id IS NULL;
 count
-------
     0
(1 row)

[ gstg ] production> Note.where(discussion_id: nil).count
=> 0

Migration output

== 20220524074947 FinalizeBackfillNullNoteDiscussionIds: migrating ============
== 20220524074947 FinalizeBackfillNullNoteDiscussionIds: migrated (0.0247s) ===

== 20220524081955 RemoveNotesNullDiscussionIdTempIndex: migrating =============
-- transaction_open?()
   -> 0.0000s
-- indexes(:notes)
   -> 0.0048s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- remove_index(:notes, {:algorithm=>:concurrently, :name=>"tmp_index_notes_on_id_where_discussion_id_is_null"})
   -> 0.0055s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20220524081955 RemoveNotesNullDiscussionIdTempIndex: migrated (0.0178s) ====

-- Current migration is skipped since it modifies '[:gitlab_main]' which is outside of '[:gitlab_ci, :gitlab_shared]'
== 20220524081955 RemoveNotesNullDiscussionIdTempIndex: migrating =============
-- transaction_open?()
   -> 0.0000s
-- indexes(:notes)
   -> 0.0096s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- remove_index(:notes, {:algorithm=>:concurrently, :name=>"tmp_index_notes_on_id_where_discussion_id_is_null"})
   -> 0.0077s
-- execute("RESET statement_timeout")
   -> 0.0003s
== 20220524081955 RemoveNotesNullDiscussionIdTempIndex: migrated (0.0230s) ====

== 20220524081955 RemoveNotesNullDiscussionIdTempIndex: reverting =============
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:notes, :id, {:where=>"discussion_id IS NULL", :name=>"tmp_index_notes_on_id_where_discussion_id_is_null", :algorithm=>:concurrently})
   -> 0.0077s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- add_index(:notes, :id, {:where=>"discussion_id IS NULL", :name=>"tmp_index_notes_on_id_where_discussion_id_is_null", :algorithm=>:concurrently})
   -> 0.0069s
-- execute("RESET statement_timeout")
   -> 0.0003s
== 20220524081955 RemoveNotesNullDiscussionIdTempIndex: reverted (0.0210s) ====

== 20220524074947 FinalizeBackfillNullNoteDiscussionIds: reverting ============
== 20220524074947 FinalizeBackfillNullNoteDiscussionIds: reverted (0.0000s) ===

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.