ci: run parametric tests with oldstable

We run parametric tests with the oldest Go language version we support.
As we're now aligning with the official Go support policy, that would
now be what the Go GitHub Actions tooling calls "oldstable".

.github/workflows/parametric-tests.yml

@@ -43,7 +43,7 @@ jobs:
 
       - uses: actions/setup-go@v3
         with:
-          go-version: "1.20"
+          go-version: "oldstable"
 
       - name: Build runner
         uses: ./.github/actions/install_runner