Merge branch '6.1' into 6.2

* 6.1:
  [HttpClient] Fix computing retry delay when using RetryableHttpClient
  [Uid] Fix validating UUID variant bits
  [Validator][UID] Stop to first ULID format violation
  [Bridge] Fix mkdir() race condition in ProxyCacheWarmer
  [Cache] update readme
  Bug #42343 [Security] Fix valid remember-me token exposure to the second consequent request
  Prevent exception if request stack is empty
  Psr18Client ignore invalid HTTP headers
  skip a transient test on AppVeyor

Author: nicolas-grekas

CacheWarmer/ProxyCacheWarmer.php

@@ -48,7 +48,7 @@ public function warmUp(string $cacheDir): array
         foreach ($this->registry->getManagers() as $em) {
             // we need the directory no matter the proxy cache generation strategy
             if (!is_dir($proxyCacheDir = $em->getConfiguration()->getProxyDir())) {
-                if (false === @mkdir($proxyCacheDir, 0777, true)) {
+                if (false === @mkdir($proxyCacheDir, 0777, true) && !is_dir($proxyCacheDir)) {
                     throw new \RuntimeException(sprintf('Unable to create the Doctrine Proxy directory "%s".', $proxyCacheDir));
                 }
             } elseif (!is_writable($proxyCacheDir)) {