* Refactor Inflexible Snapshot Repository BwC (#52365)
Transport the version to use for a snapshot instead of whether to use shard generations in the snapshots in progress entry. This allows making upcoming repository metadata changes in a flexible manner in an analogous way to how we handle serialization BwC elsewhere.
Also, exposing the version at the repository API level will make it easier to do BwC relevant changes in derived repositories like source only or encrypted.
If some internal `.watcher` or so index gets created during these tests
then the shard counts on snapshot restores and creates won't match up with expectations.
Fixed by only creating the snapshot for the test index
Closes#50819
* Fix MultiVersionRepository BwC Tests
The HLRC doesn't like what its getting back from some older 6.x versions for the restore status
so I moved that request to the low level client.
Closes#50819
Follow up to #50692 that starts writing a `min_version` field to
the `RepositoryData` so that pre-7.6 ES versions can not read it
(and potentially corrupt it if they attempt to modify the repo contents)
after the repository moved to the new metadata format.
* Fix Snapshot Repository Corruption in Downgrade Scenarios (#50692)
This PR introduces test infrastructure for downgrading a cluster while interacting with a given repository.
It fixes the fact that repository metadata in the new format could be written while there's still older snapshots in the repository that require the old-format metadata to be restorable.
Mark Vieira
Armin Braun