CLI server: document "Host" header reflection removal in UPGRADING

Author: DanielEScherzer

UPGRADING

@@ -528,6 +528,8 @@ PHP 8.6 UPGRADE NOTES
 - CLI:
   . The built-in development server now accepts requests using the HTTP QUERY
     method instead of returning 501 Not Implemented.
+  . The built-in development server no longer reflects the "Host" header from
+    requests.
 
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 4. Deprecated Functionality