wsrep_slave_threads deprecated in mySQL 8.0+

[nblount]

As wsrep_slave_threads is not set to anything on new MySQL versions it seems to guess on a value, maybe wsrep_applier_threads should be used instead when wsrep_slave_threads is not found.

Deprecated as of MySQL-wsrep 8.0.26-26.8 in favor of [wsrep_applier_threads]
https://galeracluster.com/library/documentation/mysql-wsrep-options.html#wsrep-slave-threads

[FabioPedretti]

PR #854 should have fixed this issue, can you please check and eventually close this issue? Thanks.