Use MetadataName.RetryAfter in rate limiting OnRejected example and add .NET 11 note

[Copilot]

The OnRejected example in the rate limiting article hard-codes Retry-After: 60 and doesn't mention that in .NET 11 FixedWindowRateLimiter reports an accurate RetryAfter metadata value. This PR updates the example and adds a version-scoped note.

• OnRejected example (aspnetcore/performance/rate-limit.md)

  • The hard-coded header is replaced with a read of MetadataName.RetryAfter from context.Lease. The header is set only when the limiter provides that metadata. This matches the pattern in rate-limit-samples.md.
  • A line after the example lists the required using System.Globalization; and using System.Threading.RateLimiting;.

  if (context.Lease.TryGetMetadata(MetadataName.RetryAfter, out var retryAfter))
  {
      context.HttpContext.Response.Headers.RetryAfter =
          ((int)retryAfter.TotalSeconds).ToString(NumberFormatInfo.InvariantInfo);
  }

• .NET 11 note

  • A > [!NOTE] under :::moniker range=">= aspnetcore-11.0" says fixed-window rejections now report the next window boundary. It also says limiters that can't estimate availability, such as ConcurrencyLimiter, provide no metadata.
  • The surrounding >= aspnetcore-7.0 zone is closed and reopened around the note. The article now has three moniker openers and closers.

• Metadata: ms.date is updated.

Review notes:

• The <xref:System.Threading.RateLimiting.*> UIDs (FixedWindowRateLimiter, ConcurrencyLimiter) are taken from the issue's proposed text. They haven't been checked against the API docs or the OpenPublishing build.
• The MetadataName.RetryAfter pattern is valid on earlier versions, so the example is unscoped and only the note is .NET 11-only.

• Fixes v11 update: Accurate rate-limiting Retry-After headers #37724

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