New release checklist

1. check copyright dates in doc sections in MQTTAsync.h and MQTTClient.h

2. update version number (major.minor.patch) in version files

3. check binary backward compatibility

4. merge develop branch into master

   • make sure version in version files is correct in master as well as develop

5. Update version numbers in artifact_linux.yml and artifact_macos.yml workflow files in the master branch

6. generate docs (make html) and update the docs folder

7. push all updates and make sure the builds work

8. tag the release, assuming all the previous steps have worked

9. update the release with the build artifacts

10. update the Paho website - and any other online docs