Update targeted SDKs to versions that are currently supported

[torselden]

This project currently partially targets netcoreapp3.1 and net6.0 which are no longer supported.

What area is it related to
samples- and test-projects

Describe the solution you'd like
Target the latest availale sdk - net9.0

Describe alternatives you've considered
net9.0 is a short term support version, but other than that no obvious benefit of targeting a lower version instead.

Additional context
Updating all project seems to compile fine, with only one small warning to fix.