Update postgres Docker tag to v13.20

This MR contains the following updates:

Package	Update	Change
postgres	minor	13.6 -> 13.20

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webserver/docker_keycloak/templates/docker-compose.yml

@@ -33,7 +33,7 @@ services:
 
   db:
 
-    image: postgres:13.6
+    image: postgres:13.20
     restart: always
     volumes:
       - /srv/keycloak/db:/var/lib/postgresql/data