Add explicit System.Text.Json reference to fix assembly binding

FAKE 6.1.4 dependencies require System.Text.Json 8.0.0.0, but .NET 6
ships with an older version. Adding explicit PackageReference to
System.Text.Json 8.0.5 resolves the assembly binding error.

Author: claude

fake-build/Build.fsproj

@@ -18,5 +18,6 @@
     <PackageReference Include="Fake.Core.ReleaseNotes" Version="6.1.4" />
     <PackageReference Include="Fake.DotNet.AssemblyInfoFile" Version="6.1.4" />
     <PackageReference Include="Fake.Api.GitHub" Version="6.1.4" />
+    <PackageReference Include="System.Text.Json" Version="8.0.5" />
   </ItemGroup>
 </Project>