feat: use library for decoding of payload

Author: tinect

src/Storefront/Framework/AltchaCaptcha.php

@@ -44,10 +44,7 @@ public function isValid(Request $request, array $captchaConfig): bool
         }
 
         try {
-            $decodedPayload = \base64_decode($verifyData, true);
-            $payload = \json_decode($decodedPayload, true, 512, \JSON_THROW_ON_ERROR);
-
-            return (new Altcha($secretKey))->verifySolution($payload);
+            return (new Altcha($secretKey))->verifySolution($verifyData);
         } catch (\Throwable) {
             return false;
         }