Upgrade .NET projects to C# 8.0.

Author: tritao

Directory.Build.props

@@ -16,7 +16,7 @@
     <ActionDir>$(BuildDir)$(PremakeAction)\</ActionDir>
     <NativeProjectsDir>$(ActionDir)projects\</NativeProjectsDir>
     <TargetDir>$(OutputPath)</TargetDir>
-    <LangVersion>7.3</LangVersion>
+    <LangVersion>8.0</LangVersion>
     <WarningLevel>4</WarningLevel>
     <DotNetCmd>dotnet</DotNetCmd>
     <GeneratorFileExtension>dll</GeneratorFileExtension>

src/Runtime/CppSharp.Runtime.csproj

@@ -1,5 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
+    <LangVersion>7.3</LangVersion>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <TargetFramework>netstandard2.0</TargetFramework>
     <PlatformTarget>AnyCPU</PlatformTarget>