Transition `.transform-notifications*` to be hidden indices

[gwbrown]

Now that we have hidden indices (#50452) and hidden aliases (#52547) to prevent accidental operations on semi-internal indices, new indices in the .transform-notifications* and (if we're still creating new indices in this pattern) .data-frame-notifications* patterns should be hidden indices.

Because hidden indices/aliases are still fairly readily accessible, the transition shouldn't involve too much work - in some cases the only work that's been necessary has been adding the index.hidden setting to the index template - but I'm happy to take on any work involved, and wanted to make sure we were communicating clearly on the topic. We'd like to do this before the next feature freeze if possible, and I'll put up a PR shortly with some work on the change.

/cc @hendrikmuhs since I hear you're the one to talk to about this.

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)