Add ASP.NET Core release notes for .NET 10 Preview 6

[Copilot]

This PR adds comprehensive ASP.NET Core release notes for .NET 10 Preview 6 based on the feature content provided in dotnet/AspNetCore.Docs#35602.

Changes Made

• Created /release-notes/10.0/preview/preview6/aspnetcore.md following the established format from previous preview releases
• Compiled and edited content from issue comments for clarity and simplicity
• Included all 10 major features for Preview 6:

Key Features Documented

1. Automatic eviction from memory pool - Memory pools now automatically release unused memory when applications are idle
2. Blazor WebAssembly preloading - New <LinkPreload /> component replaces link headers for better asset preloading
3. Blazor build producing javascript bundler friendly output - Support for webpack/rollup compatibility with WasmBundlerFriendlyBootConfig=true
4. Improved form validation for Blazor - Support for validating nested objects and collection items with [ValidatableType] attribute
5. NotFound works with streaming that has started - Enhanced error handling for streaming scenarios
6. Blazor diagnostics improvements - Top-level traces in Aspire dashboard and Application Insights
7. NavigationException switch behavior change - Updated default behavior and configuration switch naming
8. Add passkey support to ASP.NET Core Identity - WebAuthn and FIDO2 standards support for passwordless authentication
9. Minimal API Validation integration with IProblemDetailsService - Customizable error responses for validation
10. Unified validation APIs moved to extensions package - APIs moved to Microsoft.Extensions.Validation namespace

Content Structure

• Clear table of contents with anchor links
• Detailed feature descriptions with code examples
• "Why it matters" and "How to use" sections where applicable
• Community contributor acknowledgments
• Links to related documentation and breaking changes

The content follows the same markdown structure and writing style as existing preview release notes, ensuring consistency across the documentation.

Fixes #9964.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/dotnet/aspnetcore/pulls
  • Triggering command: curl -s REDACTED (http block)
• https://api.github.com/repos/dotnet/aspnetcore/search/issues
  • Triggering command: curl -s REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[danroth27]

Should Customizable security descriptors for HTTP.sys be in this list?

@tdykstra That was shipped in .NET 10 Preview 5

[wadepickett]

Very clean, looks good!