CI: add back in older pg versions

Author: zachdaniel

.github/workflows/elixir.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        postgres-version: ["16"]
+        postgres-version: ["14", "15", "16"]
     uses: ash-project/ash/.github/workflows/ash-ci.yml@main
     with:
       postgres: true