The script wsrep_sst_backup was introduced on MariaDB 10.3 in commit
9b2fa2ae. The new script was automatically included in RPM packages but not
in Debian packages (which started to fail on warning about stray file).

Include wsrep_sst_backup in the mariadb-server-10.5+ package, and also
include a stub man page so that packaging of a new script is complete.

Related:
https://galeracluster.com/documentation/html_docs_20210213-1355-master/documentation/backup-cluster.html

This commit was originally submitted in May 2022 in
https://github.com/MariaDB/server/pull/2129 but upstream indicated only
in May 2023 that it might get merged, thus this is for a later release.

All new code of the whole pull request, including one or several files
that are either new files or modified ones, are contributed under the
BSD-new license. I am contributing on behalf of my employer Amazon Web
Services, Inc.