Use postgres 12 on github actions

This is the version we're using on production.

.github/workflows/build-and-test.yml

@@ -9,7 +9,7 @@ jobs:
         node-version: [16.x]
     services:
       postgres:
-        image: postgres:11
+        image: postgres:12
         env:
           POSTGRES_USER: postgres
           POSTGRES_PASSWORD: postgres