Improve release workflow

[papanito]

Following improvements shall be done

• create release only when merged, not when tagged as it currently happens

  • https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#onpushpull_requestbranchestags
  • https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target

• improve release notes text:

  Currently the release notes text us automatically but sometimes we would like to add some more context. One idea is to use git tag commit text and add this to it.