Fix automatic GH release creation on release

[SylvainJuge]

The github release is not automatically published anymore, which was the cause of #239 that I fixed manually.

Fixing that would likely require to update the .ci/release.sh script to add a call to create a release using the gh command line tool like in other projects (in a similar way as it's done with elastic/apm-agent-java#3567)

While fixing this, it could also be a good opportunity to check for the actual maven artifact publication before creating the github release to be consistent with other projects.

[amannocci]

Ref: #247