feat: adds a net8 target to benefit from all the conditional compilation

Signed-off-by: Vincent Biret <[email protected]>

Author: baywet

src/Microsoft.OpenApi/Microsoft.OpenApi.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
-        <TargetFramework>netstandard2.0</TargetFramework>
+        <TargetFrameworks>netstandard2.0;net8.0</TargetFrameworks>
         <LangVersion>Latest</LangVersion>
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
         <Version>2.0.0-preview4</Version>