Use last compiler toolset version that support .NET 8 (pythonnet#2640)

Author: filmor

Directory.Build.props

@@ -12,7 +12,7 @@
   </PropertyGroup>
   <ItemGroup>
     <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
-    <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="4.14.0">
+    <PackageReference Include="Microsoft.Net.Compilers.Toolset" Version="4.12.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>