Refactor IndexMetadataUpgradeService to IndexMetadataVerifier (#67547)

This PR removes support for index metadata upgrades:
* Stop using the `index.version.upgrade` setting and deprecate it.
* Remove `MetadataIndexUpgradeService` and other references to upgrades.

In addition to supporting upgrades, `MetadataIndexUpgradeService` verified
certain aspects of the metadata, like index version compatibility. This logic
is important to keep, so `MetadataIndexUpgradeService` was reworked to
`IndexMetadataVerifier` instead of being removed completely.

Closes #66143.