ci: remove jsok/serialize-workflow-action dependency

Deployments to test servers will not be cancelled anymore,
but it is not clear if we even want it.
This setup is much simpler because it only depends
on GitHub Actions features and does not allocate
a runner just to sleep there and wait in the queue.

.github/workflows/test-and-deploy-ipv4only.yaml

@@ -19,13 +19,8 @@ jobs:
     environment:
       name: staging-ipv4.testrun.org
       url: https://staging-ipv4.testrun.org/
-    concurrency:
-      group: ci-ipv4-${{ github.workflow }}-${{ github.ref }}
-      cancel-in-progress: ${{ !contains(github.ref, '$GITHUB_REF') }}
+    concurrency: staging-ipv4.testrun.org
     steps:
-      - uses: jsok/serialize-workflow-action@515cd04c46d7ea7435c4a22a3b4419127afdefe9
-        with:
-          github-token: ${{ secrets.GITHUB_TOKEN }}
       - uses: actions/checkout@v4
 
       - name: prepare SSH