Add rate-limiting RetryAfter fixes to What's New for .NET 11 Preview 4

[Copilot]

Adds coverage of the System.Threading.RateLimiting fixes from the .NET 11 Preview 4 libraries release notes, reframed for the ASP.NET Core audience.

The source material in dotnet/core is written from the .NET libraries perspective. The include file reframes it from the middleware perspective:

• Leads with RetryAfter — the most relevant fix for ASP.NET Core developers. FixedWindowRateLimiter now reports accurate RetryAfter metadata, so the rate-limiting middleware automatically sets a meaningful Retry-After response header.
• Secondary mention of TokenBucketRateLimiter fractional permits and ChainedRateLimiter forwarding fixes.
• Preserves contributor acknowledgments for @asbjornvad and @apoorvdarshan.

Changes

• New: aspnetcore/release-notes/aspnetcore-11/includes/rate-limiting-retry-after-preview4.md
• Updated: aspnetcore/release-notes/aspnetcore-11.md — added [!INCLUDE] in Miscellaneous section, updated ms.date

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Internal previews

📄 File	🔗 Preview link
aspnetcore/release-notes/aspnetcore-11.md	aspnetcore/release-notes/aspnetcore-11

[wadepickett]

@copilot, Check against the code sample used in the source release notes content here:
https://github.com/dotnet/core/blob/main/release-notes/11.0/preview/preview4/libraries.md#rate-limiting-fixes-for-retryafter-fractional-permits-and-chainedratelimiter

Determine why the code example was not included here in the PR, and fix if needed.

[Copilot]

@copilot, Check against the code sample used in the source release notes content here:
https://github.com/dotnet/core/blob/main/release-notes/11.0/preview/preview4/libraries.md#rate-limiting-fixes-f...

The code sample was omitted in the initial commit because I focused on reframing the prose for the ASP.NET Core audience and didn't carry over the code block. Added it now in commit 1eddbe7 — it shows how a FixedWindowRateLimiter lease exposes the RetryAfter metadata after a failed acquisition.