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

The GitHub Actions Windows runner no longer has .NET 6 available, causing
all CI builds to fail with 'You must install or update .NET to run this
application'. .NET 8 is available on the runner, so updating the FAKE
build script target to net8.0 fixes the CI pipeline.

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

Author: 2 people

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" />