Backfill notes that have null discussion_id

What does this MR do and why?

Fixes missing discussion_id in notes. This was caused by a bug that has already been fixed. So we just need to fix old data.

On GitLab.com, we need to update 25M rows:

gitlabhq_dblab=# SELECT COUNT(*) FROM notes WHERE discussion_id IS NULL;
  count
----------
 25156506

Migration output

== 20220420061439 AddNotesNullDiscussionIdTempIndex: migrating ================
-- 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})
   -> 0.0237s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:notes, :id, {:where=>"discussion_id IS NULL", :name=>"tmp_index_notes_on_id_where_discussion_id_is_null", :algorithm=>:concurrently})
   -> 0.0063s
-- execute("RESET statement_timeout")
   -> 0.0005s
== 20220420061439 AddNotesNullDiscussionIdTempIndex: migrated (0.0545s) =======

== 20220420061450 BackfillNullNoteDiscussionIds: migrating ====================
== 20220420061450 BackfillNullNoteDiscussionIds: migrated (0.0164s) ===========

== 20220420061450 BackfillNullNoteDiscussionIds: reverting ====================
== 20220420061450 BackfillNullNoteDiscussionIds: reverted (0.0000s) ===========

== 20220420061439 AddNotesNullDiscussionIdTempIndex: reverting ================
-- transaction_open?()
   -> 0.0000s
-- indexes(:notes)
   -> 0.0082s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- remove_index(:notes, {:algorithm=>:concurrently, :name=>"tmp_index_notes_on_id_where_discussion_id_is_null"})
   -> 0.0033s
-- execute("RESET statement_timeout")
   -> 0.0003s
== 20220420061439 AddNotesNullDiscussionIdTempIndex: reverted (0.0166s) =======

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.

Related to #346495 (closed)