Bring back the timestamp task/batch app for 2.0.x (#192)

- Remove unused timestamp task apps `docker/assembly.xml`
- Resurrect the timestamp task/batch 2.0.x app
- Add the resurrected apps to CI which publishes them to `/milestone`

.github/workflows/ci-pr.yml

@@ -6,7 +6,7 @@ on:
 jobs:
   build:
     name: Call Build Workflow
-    uses: spring-cloud/spring-cloud-dataflow-samples/.github/workflows/ci.yml@main
+    uses: ./.github/workflows/ci.yml
     with:
       maven-build-only: true
     secrets: