ci: update build script target framework from net6.0 to net8.0

The GitHub Actions Windows runner no longer ships .NET 6 (only 8, 9, 10).
This change mirrors PR #461 to allow CI to at least reach the build stage.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

build/build.fsproj

@@ -2,7 +2,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net8.0</TargetFramework>
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="fakeiisexpress.fs" />