Expand Postgres CI matrix (#721)

Author: jechol

.github/workflows/elixir.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        postgres-version: ["14", "15", "16"]
+        postgres-version: ["14", "15", "16", "17", "18"]
     uses: ash-project/ash/.github/workflows/ash-ci.yml@main
     with:
       postgres: true