bug #18688 [HttpFoundation] Warning when request has both Forwarded and X-Forwarded-For (magnusnordlander)

This PR was squashed before being merged into the 2.7 branch (closes #18688).

Discussion
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[HttpFoundation] Warning when request has both Forwarded and X-Forwarded-For

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | symfony/symfony-docs#6526

Emit a warning when a request has both a trusted Forwarded header and a trusted X-Forwarded-For header, as this is most likely a misconfiguration which causes security issues.

Commits
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ee8842f [HttpFoundation] Warning when request has both Forwarded and X-Forwarded-For

Author: fabpot

Resources/config/web.xml

@@ -46,5 +46,9 @@
             <argument type="service" id="request_stack" />
             <tag name="kernel.event_subscriber" />
         </service>
+
+        <service id="validate_request_listener" class="Symfony\Component\HttpKernel\EventListener\ValidateRequestListener">
+            <tag name="kernel.event_subscriber" />
+        </service>
     </services>
 </container>