Update C# 15 docs for .NET 11 RC1 stabilization

[eiriktsarpalis]

Describe the issue or suggestion

.NET 11 RC1 selects C# 15 by default for net11.0. The official C# RC1 release notes identify union types, closed class hierarchies, and extension indexers as stabilized C# 15 features. They no longer require <LangVersion>preview</LangVersion>. Several docs still say otherwise:

• docs/csharp/whats-new/csharp-15.md calls C# 15 the latest preview release. Its union section also says some features are coming in future previews; check that claim against RC1.
• docs/csharp/whats-new/tutorials/unions.md calls unions a preview feature and instructs readers to set LangVersion to preview in both dotnet new commands and project files. Its prerequisite paragraph also refers to closed hierarchies rather than unions.
• docs/csharp/whats-new/tutorials/closed-hierarchies.md makes the same preview requirement and instructs readers to add a ClosedAttribute polyfill for the .NET 11 Preview 5 SDK. The current .NET 11 SDK supports closed under C# 15 without either step.
• docs/csharp/whats-new/tutorials/extension-members.md says extension indexers require the preview language setting.

Update the tutorials and their samples together. The telemetry-monitor projects under docs/csharp/whats-new/tutorials/snippets/shared/telemetry-monitor/ still target net10.0 with LangVersion=preview; simply removing that property would not make them .NET 11 examples. The PointExtensions sample project and union language-reference sample project already target net11.0 but still opt into preview syntax. Retarget or simplify the project files as appropriate, remove the now-unnecessary closed-hierarchy polyfill if the updated sample builds without it, and run the samples with the .NET 11 SDK.

Do not remove preview guidance for the separate Unsafe Evolution / updated memory safety rules feature. The RC1 notes explicitly say that feature still requires <LangVersion>preview</LangVersion> and its feature flag.