SPEED-1066: Update Speed Brain rejected response code from 503 to 406 (#18019)

This commit is changing the public documentation about Speed Brain
rejecting speculation requests with the newly decided 406 response code
instead of the current 503, which was introduced during the initial
release of the Speed Brain product during Birthday Week '24.

Author: aseure

src/content/docs/speed/optimization/content/speed-brain.mdx

@@ -55,7 +55,7 @@ The configuration looks like this:
 }
 ```
 
-This configuration instructs the browser to initiate prefetch requests for future navigations. These prefetch requests will include the `sec-purpose: prefetch` HTTP request header. Prefetches that are not successful will respond with a `503` status code. Prefetches that are successful will respond with a `200` status code.
+This configuration instructs the browser to initiate prefetch requests for future navigations. These prefetch requests will include the `sec-purpose: prefetch` HTTP request header. Prefetches that are not successful will respond with a `406` status code (used to be `503`). Prefetches that are successful will respond with a `200` status code.
 
 ## Test Speed Brain