Set default TFM for Traversal to SDK version

[MattKotsenas]

Fixes #602.

Defaulting to net45 feels odd, as it means folks are taking new dependencies on .NET Framework without realizing it. It can also lead to oddities like #602 where old defaults unexpectedly apply to newer projects.

This change allows the user to specify a particular version as before. Otherwise $(NETCoreAppMaximumVersion) becomes the default version.

I think this strikes a better balance, where users that care or need a specific version continue to do so, while those that consider Traversal projects more a facility of the SDK / build system get a more sensible default than a version released 13 years ago.