chore(deps): update mysql docker tag to v9.6

Author: renovate[bot]

docker-compose.test.yml

@@ -5,7 +5,7 @@ services:
     depends_on:
     - bookstack
   mysql:
-    image: mysql:9.5
+    image: mysql:9.6
     environment:
     - MYSQL_ROOT_PASSWORD=secret
     - MYSQL_DATABASE=bookstack

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   mysql:
-    image: mysql:9.5
+    image: mysql:9.6
     environment:
     - MYSQL_ROOT_PASSWORD=secret
     - MYSQL_DATABASE=bookstack