.NET December 2025 Updates (release-6.8)

Release a new version of ServiceControl 6.8 including the runtime from the [.NET December 2025 Updates](https://github.com/dotnet/announcements/issues/378).

There are no code changes. The new .NET Runtime is included as part of the build process.

See the [.NET Update issue](https://github.com/dotnet/announcements/issues/378) for details of what was included in the .NET release.