Replace feature::maintenance with type::maintenance

What does this MR do?

This follows the guidance that we have since we renamed ~"feature::maintenance" to typemaintenance with Consolidate top level type labels (gitlab-org&6622 - closed).

The feature::maintenance label isn't supposed to exist anymore as it's been renamed to typemaintenance (it's just re-created occasionally by automated tooling, since adding a label via the API actually creates it if it doesn't exist, which is annoying).