* Update Microsoft.Extensions.Logging to v7.0.0

* Fix nullability warnings

Now only warns about untested netcoreapp3.1 compatibility

* Update template project

And remove explicit hints that make the tests optional,
since most bindings will have some methods that are testable.

* Update TFM for tests to .NET 6.0

* Make test compile for .NET4.8

* Workaround for missing File.WriteAllTextAsync.

Causes a compatibility regression when a netcoreapp3.1 application
now reverts to use the netstandard2.0 library.

* Move samples to separate folder

(Another instance of the macos compiler failure that occurs if
two projects share the same physical folder)
Co-authored-by: NMatt Galbraith <MattGal@users.noreply.github.com>