Fix deprecation

```
Deprecation Notice: Symfony\Thanks\GitHubClient::getRepositories(): Implicitly marking parameter $failures as nullable is deprecated, the explicit nullable type must be used instead in /Users/oskar.stark/.composer/vendor/symfony/thanks/src/GitHubClient.php:109
```

I don't think we want to allow `null` here @nicolas-grekas

Author: OskarStark

src/GitHubClient.php

@@ -106,7 +106,7 @@ public function __construct(Composer $composer, IOInterface $io)
         }
     }
 
-    public function getRepositories(?array &$failures = null, $withFundingLinks = false): array
+    public function getRepositories(?array &$failures = null, bool $withFundingLinks = false): array
     {
         $repo = $this->composer->getRepositoryManager()->getLocalRepository();