Fix CI deprecation notices

[chrisib]

Remove set-output, use GITHUB_OUTPUT instead. Should resolve #172

[roryjamesallen]

I have no idea how CI works so if you do and these changes fix the notices then LGTM

[chrisib]

I noticed a few more notices with the tests for this branch, so I'll try making a few more changes before merging.

[chrisib]

As of that last push, the only error left is

 build_and_publish / build-n-publish: # >> PyPA publish to PyPI GHA: POTENTIALLY INVALID TOKEN <<#L1

It looks like you are trying to use an API token to authenticate in the package index and your token value does not start with "pypi-" as it typically should. This may cause an authentication error. Please verify that you have copied your token properly if such an error occurs.

I don't think I have access to the PyPi credentials, so @roryjamesallen if this is a significant concern I think you'll need to dig into that. But the deprecation notices are all gone, which was the main purpose of this merge request.