Fix OAuth 2.0 keys being generated with PKCS1

Closes #4223

Merge request studip/studip!3055

cli/Commands/OAuth2/Keys.php

@@ -47,8 +47,8 @@ class Keys extends Command
         $this->storeKeyContentsToFile($encryptionKey, $this->generateEncryptionKey());
 
         $keys = RSA::createKey(4096);
-        $this->storeKeyContentsToFile($publicKey, $keys->getPublicKey()->toString('PKCS1'));
-        $this->storeKeyContentsToFile($privateKey, $keys->toString('PKCS1'));
+        $this->storeKeyContentsToFile($publicKey, (string) $key->getPublicKey());
+        $this->storeKeyContentsToFile($privateKey, (string) $key);
 
         $io->info('Schlüsseldateien erfolgreich angelegt.');