feat: Use C# 8.0 by default

BREAKING CHANGE: Uno is now built using VS2019 16.4

It does not support building Xamarin iOS/Android applications with Visual Studio 2017.

Linker errors may occur such as:
```
Java.Interop.Tools.Diagnostics.XamarinAndroidException: error XA2006: Could not resolve reference to 'System.Collections.Generic.Queue`1'
(defined in assembly 'Uno.UI, Version=255.255.255.255, Culture=neutral, PublicKeyToken=null') with scope 'mscorlib, Version=2.0.5.0,
Culture=neutral, PublicKeyToken=7cec85d7bea7798e'.
```

src/Directory.Build.props

@@ -21,7 +21,7 @@
     <CreateHardLinksForAdditionalFilesIfPossible>true</CreateHardLinksForAdditionalFilesIfPossible>
     <CreateHardLinksForPublishFilesIfPossible>true</CreateHardLinksForPublishFilesIfPossible>
 
-    <LangVersion>7.3</LangVersion>
+    <LangVersion>8.0</LangVersion>
 
     <!-- Version management is now done through NuGet, this avoids issues related version mismatch -->
     <Version>255.255.255.255</Version>