RecalculateProductivityAnalytics migration fails with legacy DiffNote

https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/966411/

ActiveRecord::SubclassNotFound: Invalid single-table inheritance type: DiffNote is not a subclass of Gitlab::BackgroundMigration::RecalculateProductivityAnalytics::Migratable::Note
  active_record/inheritance.rb:233:in `find_sti_class'
    raise SubclassNotFound, "Invalid single-table inheritance type: #{subclass.name} is not a subclass of #{name}"
  active_record/inheritance.rb:207:in `discriminate_class_for_record'
    find_sti_class(record[inheritance_column])
  active_record/persistence.rb:69:in `instantiate'
    klass = discriminate_class_for_record(attributes)
  active_record/querying.rb:52:in `block (2 levels) in find_by_sql'
    result_set.map { |record| instantiate(record, column_types, &block) }
  active_record/result.rb:57:in `block in each'
    hash_rows.each { |row| yield row }
...
(94 additional frame(s) were not displayed)

Invalid single-table inheritance type: DiffNote is not a subclass of Gitlab::BackgroundMigration::RecalculateProductivityAnalytics::Migratable::Note