ci: avoid running pg18 tests twice

We already test PG18, the default, with the coverage job, so why run it
twice?

Author: wolfgangwalther

.github/workflows/test.yaml

@@ -70,7 +70,8 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        pgVersion: [14, 15, 16, 17, 18]
+        # Latest version is tested via `coverage` above.
+        pgVersion: [14, 15, 16, 17]
     name: PG ${{ matrix.pgVersion }}
     runs-on: ubuntu-24.04
     defaults: