build(gpu): target net8.0

OpenNest.Gpu uses no System.Drawing, P/Invoke or Windows API: PartBitmap is a
plain int grid and ILGPU's CPU, CUDA and OpenCL backends are cross-platform.
GpuSlideContactTests pass on Linux against the net8.0 build.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Gpu/OpenNest.Gpu.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFramework>net8.0-windows</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
     <RootNamespace>OpenNest.Gpu</RootNamespace>
     <AssemblyName>OpenNest.Gpu</AssemblyName>
   </PropertyGroup>