Change TargetFramework to net8.0 with inheritance

Updated TargetFramework to inherit from UnrealEngine.csproj.props with a fallback for engines that don't define it.

Also removed the explicit Microsoft.CSharp PackageReference: newer frameworks include it by default and the explicit reference triggers NU1510, which TreatWarningsAsErrors turns into an error.

SourceCodeGenerator.ubtplugin.csproj

@@ -2,7 +2,8 @@
   <Import Project="SourceCodeGenerator.ubtplugin.csproj.props" Condition="Exists('SourceCodeGenerator.ubtplugin.csproj.props')" />
   <Import Project="$(EngineDir)\Source\Programs\Shared\UnrealEngine.csproj.props" />
   <PropertyGroup>
-    <TargetFramework>net8.0</TargetFramework>
+    <!-- Inherit TargetFramework from the engine's UnrealEngine.csproj.props; fallback only for engines that don't define it -->
+    <TargetFramework Condition="'$(TargetFramework)' == ''">net8.0</TargetFramework>
     <Configuration Condition=" '$(Configuration)' == '' ">Development</Configuration>
     <OutputType>Library</OutputType>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
@@ -29,9 +30,6 @@
     <DebugSymbols>true</DebugSymbols>
     <DebugType>full</DebugType>
   </PropertyGroup>
-  <ItemGroup>
-    <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
-  </ItemGroup>
   <ItemGroup>
     <Reference Include="EpicGames.Build">
       <HintPath>$(EngineDir)\Binaries\DotNET\UnrealBuildTool\EpicGames.Build.dll</HintPath>