Close #529

this adds a build check for CI

multi targeting new and legacy TFMs is not supported on mac,
so we don't build net6 TFMs on mac and with our samples having project references,
this fails the samples build.

net6.0 warns that this is always false...

does not occur anymore

These changes aim to keep compatibility with legacy TFMs while being net6 friendly.
The Android binding project TFM target has been bumped as well as UAP targets.

also always make sure NuGet is available