Bump local dev db to postgresql 15 to match prod (#1585)

Note this will require something like `docker volume rm shipit_dbstore`
in local dev environments, since the db container will refuse to start
against an old datadir.

docker-compose.yml

@@ -67,7 +67,7 @@ services:
       - api
 
   db:
-    image: postgres:11
+    image: postgres:15
     ports:
       - "5432:5432"
     volumes: