Improve logic for default target framework version for msbuild

[belav]

The logic in CSharpier.MsBuild.targets will need to be updated every time a new .NET version comes up.

It should be possible to make use of something like this.

$([MSBuild]::VersionGreaterThanOrEquals($(_TargetFrameworkVersionWithoutV), '6.0'))

It will have to take into account the ability to override via CSharpier_FrameworkVersion

It is also not clear why this was required when adding net8 - CSharpier_FrameworkVersion=$(CSharpier_FrameworkVersion). Figure out if there is a better way to deal with this all. Right now a lot of it feels like a kludge.

If possible get some more test scenarios. Right now the only scenario uses net8.