Convert upgrade check from gitlab-CI scriptish to python
As per https://gitlab.freedesktop.org/freedesktop/helm-gitlab-omnibus/-/merge_requests/8#note_509875 discussion:
Right, that's what I meant: that it was easier (given shell) to write YAML and then convert YAML into JSON. But might the overall script be easier to deal with and more simple to follow/extend if it was converted to Python, where it would be much easier to process YAML and JSON? That means we wouldn't spend half the script just doing grep/cut/etc.
So we should convert the job to a python script. This will also help when manually checking the release.