Add closed PRs for the miestone to the release notes

Adds an entry for each closed PR in the release milestone to the release notes in the
format:
```
<Title> by @ in https://github.com/spring-cloud/spring-cloud-dataflow/pull/
```

Author: Corneil du Plessis

Diff for: .github/workflows/github-release.yml

@@ -41,13 +41,15 @@ jobs:
           GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}
         run: |
           # generate release notes
-          .github/workflows/generate-release-notes.sh "${{ runner.temp }}" "$BUILD_ZOO_HANDLER_spring_cloud_deployer_version" "$BUILD_ZOO_HANDLER_spring_cloud_dataflow_ui_version" "$BUILD_ZOO_HANDLER_spring_cloud_dataflow_version" 
+          .github/workflows/generate-release-notes.sh "${{ runner.temp }}" "$BUILD_ZOO_HANDLER_spring_cloud_deployer_version" "$BUILD_ZOO_HANDLER_spring_cloud_dataflow_ui_version" "$BUILD_ZOO_HANDLER_spring_cloud_dataflow_version"
+          export PACKAGE_VERSION=${BUILD_ZOO_HANDLER_spring_cloud_dataflow_version}
+          echo "## What's Changed in Spring Cloud Dataflow and Skipper from `v<previous-version>` to `v${PACKAGE_VERSION}` >> "${{runner.temp}}/release_notes.md"
+          gh pr list --search "milestone:$PACKAGE_VERSION" --state closed --json=number,title,author --jq '.[] | "* " + .title + " by @" + .author.login + " in https://github.com/spring-cloud/spring-cloud-dataflow/pull/" + (.number|tostring)' >> "${{runner.temp}}/release_notes.md"  
           gh release create v$BUILD_ZOO_HANDLER_spring_cloud_dataflow_version \
             --draft \
             --title "Spring Cloud Data Flow $BUILD_ZOO_HANDLER_spring_cloud_dataflow_version" \
             --notes-file "${{runner.temp}}/release_notes.md"
           # retrieve spring-cloud-dataflow-package and upload to github release
-          export PACKAGE_VERSION=${BUILD_ZOO_HANDLER_spring_cloud_dataflow_version}
           ./mvnw -s .settings.xml build-helper:parse-version versions:set -DskipResolution=true -DprocessAllModules=true -DgenerateBackupPoms=false -Dartifactory.publish.artifacts=false -DnewVersion=$PACKAGE_VERSION  -B --no-transfer-progress
           ./spring-cloud-dataflow-package/set-package-version.sh
           ./mvnw -s .settings.xml package -pl spring-cloud-dataflow-package  -B --no-transfer-progress