WPF projects that use a versioned custom SDK fail to build (#5426)

* Fix build when using and SDK that comes from a PackageReference

* Update custom Sdk attribute version parsing to match msbuild

* Use SdkReference.TryParse

Co-authored-by: Ryland <41491307+ryalanms@users.noreply.github.com>

Author: ThomasGoulet73