Adjust wsrep_sst_mariabackup.sh to understand both pre-MDEV-18931 and post-MDEV-18931 file naming

The patch for MDEV-18931 introduced a new naming for MariaDB backup files.

In this task we'll additionally fix wsrep_sst_mariabackup.sh to support SST for multi-version environment, when SST is done from an old (pre-MDEV-18931) node to a new (post-MDEV-18931) node.

A possible solution (proposed by sysprg and serg) would be to check (before doing the actual job) the backup directory for old files and rename them (if exit) according to the new naming, e.g. like this:

[ -f "$DATA/xtrabackup_galera_info" ] && mv "$DATA/xtrabackup_galera_info" "$DATA/mariadb_backup_galera_info"

(and similar renames for other essential files).