Upgrade to Laravel 9 (#4413)

Co-authored-by: DaneEveritt <dane@daneeveritt.com>

app/Transformers/Api/Client/UserSSHKeyTransformer.php

@@ -20,7 +20,7 @@ class UserSSHKeyTransformer extends BaseClientTransformer
             'name' => $model->name,
             'fingerprint' => $model->fingerprint,
             'public_key' => $model->public_key,
-            'created_at' => $model->created_at->toIso8601String(),
+            'created_at' => $model->created_at->toAtomString(),
         ];
     }
 }