Update test library project to .net 6.0

[mnieto]

PR Summary

Update test library project to .net 6.0.

Builds fail in Github actions because windows-latest no longer contains netcore 3.1 SDK.

PR Checklist

First 2 checks don't apply

• [ ] Write Help
• [ ] Write Unit Test
• [x] Update CHANGELOG.md

[mnieto]

cancelling PR because 2 different topics in the same PR, will create separate PRs for each