.NET modernization, second pass

[JeremyCaney]

After the initial upgrade to a .NET 8 floor with C# 14 (#114, #115), I had done an initial pass of modernizing the code. That covered most everything that was non-breaking. Since we're committed to a major release (6.0), this takes on the bigger items: Dropping the deprecated [obsolete] layer, sealing public classes by default, and adopting TimeProvider for deterministic time-based testing.

Tasks

• Delete the OnTopic/Obsolete/ folder
• Inject TimeProvider into the types that stamp time, replacing DateTime.UtcNow and DateTime.Now
  • TopicRepository.Save()
  • TrackedRecord<T>.LastModified
  • Version guards in SqlTopicRepository.Load(), Refresh() and CachedTopicRepository.Load()
• Seal public concrete classes by default; curate and unseal only the confirmed extension points
  • View Components
  • Controllers
  • Navigation base types