Update test code to .net 6

[kmahone]

Our test code currently uses .net 2.1 which is end of life.

PR #6486 updates the test code to .net 3.1 which is still in support through the end of 2022.

To stay current we need to move to .net 6.

[marcelwgn]

What steps need to be taken for this? Is it just bumping the .NET version to 6.0 or would this require other things (such as updating build agents etc.)?

[kmahone]

I don't know exactly what is required here, but it is more involved than just updating some versions numbers since there are some differences in the msbuild project files for .net 5/6.

[github-actions]

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