Merge branch '4.4'

* 4.4:
  [Debug] Improve UPGRADE files
  remove wrongly added legacy group from test
  consistently throw NotSupportException
  [HttpKernel] Clarify error handler restoring process again
  [HttpClient] Remove CURLOPT_CONNECTTIMEOUT_MS curl opt
  add missing conflict rule
  [Intl] fix nullable phpdocs and useless method visibility of internal class
  remove some more useless phpdocs
  Resilience against file_get_contents() race conditions.
  Turned return type annotations of private methods into php return types.

Author: nicolas-grekas

Filesystem.php

@@ -281,11 +281,9 @@ public function rename(string $origin, string $target, bool $overwrite = false)
     /**
      * Tells whether a file exists and is readable.
      *
-     * @return bool
-     *
      * @throws IOException When windows path is longer than 258 characters
      */
-    private function isReadable(string $filename)
+    private function isReadable(string $filename): bool
     {
         $maxPathLength = PHP_MAXPATHLEN - 2;