We use the wsrep_notify.sh script to notify changes in Galera cluster
membership to haproxy. When xtrabackup was used for the state transfer,
nodes in the Donor state would be included in the backend pool. However,
since the switch to mariabackup in the Stein cycle, we now remove nodes
in the Donor state from the backend pool.

This change ensures that nodes in the Donor state are included in the
backend pool when the SST method is either xtrabackup or mariabackup.

https://galeracluster.com/library/documentation/mysql-wsrep-options.html#wsrep-notify-cmd



Change-Id: Ide4301779a0d221ae5d4dbdd4873fb8a40eb7297
Co-authored-by: Radosław Piliszek <radoslaw.piliszek@gmail.com>
Closes-Bug: #1850945