minor #17187 [Cache] Add return hint (mohamedGasmii)

This PR was merged into the 6.0 branch.

Discussion
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[Cache] Add return hint

Return hint for invalidate added in V6.0

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Commits
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d3b56df [Cache] Add return hint

Author: xabbuh

http_cache/cache_invalidation.rst

@@ -60,7 +60,7 @@ to support the ``PURGE`` HTTP method::
 
     class CacheKernel extends HttpCache
     {
-        protected function invalidate(Request $request, bool $catch = false)
+        protected function invalidate(Request $request, bool $catch = false): Response
         {
             if ('PURGE' !== $request->getMethod()) {
                 return parent::invalidate($request, $catch);