Dont allow to backup without schema

[Michal-Leszczynski]

For Scylla versions >= 6.0/2024.2 SM backups and restores schema from the output of DESC SCHEMA WITH INTERNALS cql query. This means that schema can be backed up as CQL file only when SM has CQL credentials to the cluster. This results in backups of Scylla 6.0/2024.2 not containing the schema required for restore purposes - SM allows for such backups to proceed because:

• in previous Scylla versions the CQL credentials weren't needed for the backup to finish
• there is a workaround to restore schema manually (SM still backs up schema sstables and they can be uploaded to a single node cluster in order to run DESC SCHEMA WITH INTERNALS on it)

This is messy, so SM shouldn't allow for a backup not containing proper schema. It should require CQL creds for Scylla versions >= 6.0/2024.2 for running the backup (in case user would really like to run a backup without CQL creds, we could add a flag like --dont-backup-schema that allows to omit schema backup when deliberately set by the user).

#3831 could make things easier as it would allow for SM to obtain the output of DESC SCHEMA WITH INTERNALS without the need for CQL creds (when the maintenance socket is open).

cc: @karol-kokoszka @pdbossman