wikireplicas: Set innodb_purge_threads to 10

labsdb1011 is struggling to keep up with innodb updates. Increase
the parallelism on purges so it can happen on 10 parallel threads.
Otherwise, it is so loaded, that single thread cannot handle it,
leading to 1 billion of pending purges.

Bug: T247978
Change-Id: I1e782844594760c79f2b07be277f21779999977c

modules/role/templates/mariadb/mysqld_config/labsdb-replica.my.cnf.erb

@@ -81,6 +81,7 @@ innodb_stats_sample_pages      = 16
 innodb_stats_method            = nulls_unequal
 innodb_file_format             = barracuda
 innodb_strict_mode             = 1
+innodb_purge_threads           = 10
 
 # dump and load innodb buffer at start and stop
 innodb_buffer_pool_load_at_startup  = 1