Use postgreSQL 14 as postgres Docker image

PostgreSQL 13 will reach its end-of-life on November 13, 2025. So we're
upgrading before that happens.

We're also upgrading to PostgreSQL 14 in our CI. In this case, we're
using the default distribution (Trixie, as of October 2025); it doesn't
affect the development or production environments, so it's OK if use the
default one.

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
     timeout-minutes: 60
     services:
       postgres:
-        image: postgres:13.16
+        image: postgres:14.19
         ports: ["5432:5432"]
         options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5
         env: