The use of the xtrabackup and xtrabackup-v2 methods for SST
has been declared obsolete since version 10.2, now it cannot
be used because of the different redo log format. Accordingly,
we need to remove the xtrabackup-related scripts and dynamically
replace the call to xtrabackup[-v2] to the mariabackup (with a
corresponding warning in the log) when the server performs SST.

https://jira.mariadb.org/browse/MDEV-17835