Document .NET 10 Preview 7 breaking change: Deprecation of WithOpenApi extension method

[Copilot]

This PR documents the breaking change announced in aspnet/Announcements#519 where the WithOpenApi extension methods in Microsoft.AspNetCore.OpenApi.OpenApiEndpointConventionBuilderExtensions have been deprecated in .NET 10 Preview 7.

Breaking Change Summary

• Type: Behavioral change (new compiler warning ASPDEPR002)
• Version: .NET 10 Preview 7
• Impact: Calls to .WithOpenApi() now produce deprecation warnings
• Reason: Duplicated functionality, simplifying API surface
• Migration: Remove .WithOpenApi() calls and use AddOpenApiOperationTransformer instead

Changes Made

1. Created new breaking change document at docs/core/compatibility/aspnet-core/10.0/withopenapi-deprecated.md following the established template
2. Added ASP.NET Core section to docs/core/compatibility/10.0.md with the new breaking change entry
3. Updated TOC at docs/core/compatibility/toc.yml to include the new document

The document includes:

• Clear description of the behavioral change
• Code examples showing before/after behavior
• Migration guidance with updated code samples
• Proper cross-references and formatting

All changes have been validated with markdownlint and follow the repository's documentation standards.

Fixes #47448.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/aspnet-core/10/withopenapi-deprecated.md	Deprecation of WithOpenApi extension method
docs/core/compatibility/toc.yml	docs/core/compatibility/toc