minor #18152 [Security] fix missing 'private' declaration (tacman)

This PR was submitted for the 6.3 branch but it was merged into the 6.2 branch instead.

Discussion
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[Security] fix missing 'private' declaration

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Commits
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83d751e fix missing 'private' declaration

Author: javiereguiluz

security/access_denied_handler.rst

@@ -34,7 +34,7 @@ unauthenticated user tries to access a protected resource::
     class AuthenticationEntryPoint implements AuthenticationEntryPointInterface
     {
         public function __construct(
-            UrlGeneratorInterface $urlGenerator,
+            private UrlGeneratorInterface $urlGenerator,
         ) {
         }