mariabackup --help output should mention that --compress is deprecated

The Knowledgebase documentation has listed the --compress option as deprecated for quite a while now as the underlying quickpress library and qpress tool have basically been unmaintained for over a decade by now.

The --help output should also show this information, and we should probably also emit a deprecation warning in the log output whenever this option is still used.

Compress still works as we use the library for that, but the corresponding --decompress option relies on the external qpress tool, for which we only provide binary packages in our Tools repositories for older distribution versions like e.g. RHEL7.x, but no longer for more current ones like e.g. RHEL 8.x.

So users on such more current platforms may end up with compressed backups they can't easily decompress.