Change test project TFM to net 6

[andrei-epure-sonarsource]

Follow-up of #1164 and #1218

See broader context in #1155 - code coverage was only working when the TFM of the test project was net48.

Now that we use coverlet, we should verify that using .net 6 in the tests doesn't affect the coverage. Instructions to test: https://github.com/SonarSource/sonar-scanner-msbuild/issues/1155#issuecomment-1024058215

• [ ] update test project to .NET 6
• [ ] update the AZP YAML file because the target framework embedded in the coverage file name

[pavel-mikula-sonarsource]

While doing this, we need to change the TFM also here: https://github.com/SonarSource/sonar-scanner-msbuild/blob/3719363458049f06a6eb8b54349a061b39c10ba3/Tests/Directory.Build.targets#L12

[pavel-mikula-sonarsource]

This is not doable before MMF-2918 (reducing S4NET versions).

We will need to remove NetFrameworkBootstrapperClass and FrameworkVersionProvider and the warnings they produce.