ColorPlugin does not work for WPF with net6 (but works for net48)
## 🐛 Bug Report

<!--- Summary description of the bug --->
After migrating a WPF project from net48 to net6 the ColorPlugin stopped working - just returns `{{DependencyProperty.UnsetValue}}` from `Convert`. The `value` passed into the `Convert` is not null of course.

### Expected behavior
ColorPlugin should work in the same way as for net48.

### Reproduction steps
1. Take sample project 
[TipCalc.zip](https://github.com/MvvmCross/MvvmCross/files/9158104/TipCalc.zip)
2. Run project with net48 and observe red `TextBlock` under the "Tip to leave" (converter working)
3. Run project with net6 and see no red color on TextBlock - converter does not work

### Configuration

**Version:** 8.0.2 (MvvmCross 8.0.2 + MvvmCross.Plugin.Color 8.0.2)

**Platform:** 
- [ ] :iphone: iOS
- [ ] :robot: Android
- [x] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms

**Additional information**
After quick research I found that `IMvxNativeColor` is not registered for `net6` build, and is registered for `net48`