Fix protobuf CI publish jobs

Part of #3 (closed)

See: #3 (comment 806532)

Attempts to fix:

• The protobuf:publish:python:gitlab job is not running
• The python:publish-to-gitlab is running where it shouldn't

And a workaround for:

• In the GitLab release, the version number of the message schemas python library does not match the version number of the release

By switching to the release-it merge request flow.