gerrit: Update mariadb to 10.6

As part of the Gerrit 3.5 upgrade we are also upgrading the reviewdb
to the latest mariadb LTS.  This should be merged after the update
process; see

  https://etherpad.opendev.org/p/gerrit-upgrade-3.5

Change-Id: Ie30c84eeb003ee86a7a66e0c1c5fd7f95ddf3f5f

playbooks/roles/gerrit/templates/docker-compose.yaml.j2

@@ -1,7 +1,7 @@
 version: '2'
 services:
   mariadb:
-    image: docker.io/library/mariadb:10.4
+    image: docker.io/library/mariadb:10.6
     network_mode: host
     environment:
       MYSQL_ROOT_PASSWORD: "{{ gerrit_reviewdb_mariadb_password }}"