don't pin digests anymore

The benefit of digest pinning isn't that great for this project really
and it comes at the cost of more issues and additional renovate noise,
so just don't anymore.
Adjust renovate config accordingly as well.

resources/chaosknoten/keycloak/docker_compose/compose.yaml.j2

@@ -46,7 +46,7 @@ services:
       - "8080:8080"
 
   db:
-    image: docker.io/library/postgres:15.14@sha256:424e79b81868f5fc5cf515eaeac69d288692ebcca7db86d98f91b50d4bce64bb
+    image: docker.io/library/postgres:15.14
     restart: unless-stopped
     networks:
       - keycloak