Switch to simple upstream releases

This commit changes our release process from the model of having a
release commit (and pull request) which also updated the NEWS.md file
and bumped the versions in the osbuild.spec and setup.py files to simply
pushing a tag.

After the tag (containing the release notes) is pushed, a GitHub
composite action is triggered that creates a GitHub release with the
contents of the git release tag. Furthermore the bumping of the version
number now always has to happen directly after a release to avoid having
to push a(n untested) commit to main for the release and this is also
handled by the GitHub composite action.

Finally packit pushes directly to dist-git now on pushing the release
tag, so no pull-request needs to be reviewed and merged anymore.

README.md

@@ -16,6 +16,7 @@ of the pipeline description, and more.
  * **Website**: <https://www.osbuild.org>
  * **Bug Tracker**: <https://github.com/osbuild/osbuild/issues>
  * **IRC**: #osbuild on [Libera.Chat](https://libera.chat/)
+ * **Changelog**: <https://github.com/osbuild/osbuild/releases>
 
 #### Contributing