[matrix] Bump postgre to 13.9

matrix/docker-compose.yml

@@ -39,7 +39,7 @@ services:
       - "traefik.http.routers.synapse-https.tls.certresolver=myhttpchallenge"
 
   db:
-    image: postgres:13.4-alpine
+    image: postgres:13.9-alpine
     env_file: ./secrets/db.secrets
     environment:
       - LC_COLLATE=C