Adding net6.0-windows for WinForms and WPF (#244)

Adding net6.0-windows for WinForms and WPF to be regularly tested instead of netcoreapp3.0 which is out-of-support and can't be run on ARM64 anymore. Unlike net462 where assemblies can be simply copied over to arm64, net6.0 requires custom built assemblies for arm64, for example by running dotnet publish --framework net6.0-windows -r win-arm64 --self-contained true. This will be followed with updating our vendor tests instructions on how to test net6.0-windows instead of netcoreapp3.0 on arm64.

Author: oggy22

SampleApps/WebView2WindowsFormsBrowser/WebView2WindowsFormsBrowser.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFrameworks>netcoreapp3.0;net462</TargetFrameworks>
+    <TargetFrameworks>netcoreapp3.0;net462;net6.0-windows</TargetFrameworks>
     <UseWindowsForms>true</UseWindowsForms>
     <Company>Microsoft</Company>
     <Authors>Hybrid Application Team</Authors>

SampleApps/WebView2WpfBrowser/WebView2WpfBrowser.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
   <PropertyGroup>
     <OutputType>WinExe</OutputType>
-    <TargetFrameworks>netcoreapp3.0;net462</TargetFrameworks>
+    <TargetFrameworks>netcoreapp3.0;net462;net6.0-windows</TargetFrameworks>
     <UseWPF>true</UseWPF>
     <Authors>Hybrid Application Team</Authors>
     <Company>Microsoft</Company>