Updates libraries to netstandard2.0 / net6.0 as appropriate

[dixonte]

Also updated dependencies to the latest-ish version. For some reason Castle.Windsor 5.1.2 seems to require Castle.Core 5 or later, but the package is set to pull in <5. Replaced the Xaml behavior reference with a modern one from MS. Haven't given this more than a "Yep, it compiles, and the demos run" test because I was planning on testing it in a dependent project and it turns out it needs the old version. Updating that will take less time than updating a project that needs rewriting anyway.