Fix reindexing retry for the notes migration

What does this MR do?

This is a follow-up from !53013 (merged). I noticed that after a reindexing failure the notes migration doesn't retry, but keeps checking the same task_id since we keep the same one in https://gitlab.com/gitlab-org/gitlab/blob/master/ee/elastic/migrate/20210302104500_migrate_notes_to_separate_index.rb#L77 when we should have set it to nil to start another reindexing for that slice.

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Related to #297646 (closed)