Upgrade Gerrit's backend database to MariaDB 10.11

This change won't automatically perform the upgrade because we don't let
ansible manage Gerrit's containers directly. But it will update the
docker-compose file for us so that we can manually pull images, down,
then up the containers again which will do the upgrade for us.

This SQL database behind Gerrit only keeps tracks of which files users
have reviewed so its importance is minimal. Getting it updated so we can
bother even less about it is a good thing.

Change-Id: I78b683770496bb3d8e97464ddedaf813780a2a4e

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

@@ -1,13 +1,14 @@
 version: '2'
 services:
   mariadb:
-    image: docker.io/library/mariadb:10.6
+    image: docker.io/library/mariadb:10.11
     network_mode: host
     environment:
       MYSQL_ROOT_PASSWORD: "{{ gerrit_reviewdb_mariadb_password }}"
       MYSQL_DATABASE: "{{ gerrit_reviewdb_mariadb_dbname }}"
       MYSQL_USER: "{{ gerrit_reviewdb_mariadb_username }}"
       MYSQL_PASSWORD: "{{ gerrit_reviewdb_mariadb_password }}"
+      MARIADB_AUTO_UPGRADE: 1
     volumes:
       # NOTE(ianw) : mounted under /home/gerrit2 (rather than more
       # usual /var/ in our configs) to keep everything together on the