Run dev environment on "slightly" newer PostgreSQL

While the current version of NAV may require no newer PostgreSQL syntax
than 9.6, that version is hopelessly outdated (and newer Django versions
we are considering moving to will not support PostgreSQLs this old).

We also run mostly version 13 in production, and without this change,
dumping and loading data from an actual production server just becomes
that much harder (dump from PostgreSQL 13 don't seem to be compatible
with 9.6).

Author: lunkwill42

docker-compose.yml

@@ -77,7 +77,7 @@ services:
     restart: on-failure:5
 
   postgres:
-    image: "postgres:9.6"
+    image: "postgres:13"
     environment:
       - POSTGRES_HOST_AUTH_METHOD=trust
     restart: on-failure:5