[ML] Remove the ML Metadata Migrator

[davidkyle]

Prior to 6.6 anomaly detector jobs were defined in the cluster state metadata, after 6.6 all new jobs were stored in the index and extant jobs automatically migrated from cluster state to index. For those interested in the glorious history it is all in #32905.

In 8.0 all jobs must have been migrated as you cannot get to 8 without passing through 7 so the migration code and checks can be removed. This must be done before the release of 8.0.0.

The main classes to remove are MlConfigMigrator, MlConfigMigrationEligibilityCheck and the associated tests once that is done it should be possible to remove MlMetadata.

[elasticmachine]

Pinging @elastic/ml-core (:ml)

[droberts195]

Closed by #80075