Update WarningLevel range (#5666)
Update WarningLevel range (#5666)

The C# 9.0 compiler has new warnings with warning level = 5.
See https://github.com/dotnet/roslyn/blob/b07cd9d710b3933d218a732b90287553ab0b8c72/docs/compilers/CSharp/Warnversion%20Warning%20Waves.md

mono/msbuild,mono/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,mono/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,cdmihai/msbuild,mono/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,AndyGerlicher/msbuild,sean-gilliam/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,cdmihai/msbuild,cdmihai/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild