minor #40775 [HttpClient] Revert "minor #40768 [PHPDoc] Fix 2 remaining return mixed (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
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[HttpClient] Revert "minor #40768  [PHPDoc] Fix 2 remaining return mixed

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

This reverts commit 40d7315202, reversing
changes made to 4e904ec108.

As spotted by the CI and reported in https://github.com/symfony/symfony/pull/40768#issuecomment-817388657

Using `Psr7ResponseInterface|mixed` instead of just `mixed` provides a better DX, by making autocompletion more useful.

Commits
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afea0a0dc2 Revert "minor #40768 [HttpClient] [PHPDoc] Fix 2 remaining return mixed (fancyweb)"

src/Symfony/Component/HttpClient/Response/HttplugPromise.php

@@ -49,8 +49,10 @@ final class HttplugPromise implements HttplugPromiseInterface
 
     /**
      * {@inheritdoc}
+     *
+     * @return Psr7ResponseInterface|mixed
      */
-    public function wait($unwrap = true): ?Psr7ResponseInterface
+    public function wait($unwrap = true)
     {
         $result = $this->promise->wait($unwrap);