ci: fix testing workflow

[matthewelwell]

Ok, so this PR was born out of issues seen primarily in #130.

I'm still not entirely sure why the workflows suddenly stopped working, but I suspect it has something to do with an upgrade on the github side which removed .net6.0 from the machines as a default.

This PR does the following:

1. Adds .net6.0 explicitly to the test workflow alongside the matrix version. I actually am not sure at this point which version the tests are actually run against based on this, but it gets them passing for now.
2. After doing this, I had to remove the global.json, and hence update release.yml. I can't really see how the global.json file ever worked with the matrix logic for the test workflow.

We should, in the near future, drop support for .net6.0 and major version the client, but I first want to get #130 out of the door and this is a quick win to get that done.