[ML] Add the index.hidden setting to ML hidden indices created before 7.7

[droberts195]

After #52420 ML is in the situation where ML internal indices created in 7.7 or above that will not become system indices will be hidden indices.

However, ML indices created before 7.7 will not currently become hidden indices.

#52772 adds the possibility to make pre-existing indices into hidden indices. Therefore, ML should add the index.hidden setting to ML indices matching the appropriate patterns. This would transition old indices, and also reintroduce the setting if it somehow got lost. It could be done by a cluster state observer on the master node.

(This change is not desperately urgent - whether it goes into 7.7 or 7.8 doesn't matter very much.)

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[droberts195]

Superseded by #53674