Migrate target framework to .NET 8 (#388)

***NO_CI***

Author: josesimoes

nanoFramework.Tools.DebugLibrary.Net/nanoFramework.Tools.DebugLibrary.Net.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>library</OutputType>
-    <TargetFrameworks>net6.0;net472</TargetFrameworks>
+    <TargetFrameworks>net8.0;net472</TargetFrameworks>
     <PlatformTarget>anycpu</PlatformTarget>
     <PackageOutputPath>../</PackageOutputPath>
     <LangVersion>latest</LangVersion>