MDEV-30822: Adjust wsrep_sst_mariabackup.sh to understand both pre-MDEV-18931 and post-MDEV-18931 file naming

This commit adds support for legacy names for files such
as mariadb_backup_galera_info, mariadb_backup_checkpoints
and mariabackup_binlog_info to allow upgrading from old
to new server versions without stopping the galera cluster.