Add temp index for notes without discussion_id

This will be used for the background migration to backfill these rows

Changelog: other

db/post_migrate/20220331074722_add_notes_null_discussion_id_temp_index_async.rb

+# frozen_string_literal: true
+
+class AddNotesNullDiscussionIdTempIndexAsync < Gitlab::Database::Migration[1.0]
+  # Temporary index to be removed in 15.0 https://gitlab.com/gitlab-org/gitlab/-/issues/357581
+  INDEX_NAME = 'tmp_index_notes_on_id_where_discussion_id_is_null'
+
+  def up
+    prepare_async_index :notes, :id, where: 'discussion_id IS NULL', name: INDEX_NAME
+  end
+
+  def down
+    unprepare_async_index_by_name :notes, INDEX_NAME
+  end
+end

db/schema_migrations/20220331074722

+507f2d64f0810938aba75f993bb7d6216499a754652b0b4c0ee2d0d44fdc3af3
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