Seedless onboarding controller: Remove usage of Buffer (#6140)

## Explanation

Remove usage of `Buffer` from `seedless-onboarding-controller`.

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## Checklist

- [x] I've updated the test suite for new or updated code as appropriate
- [x] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [x] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/contributing.md#updating-changelogs),
highlighting breaking changes as necessary
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packages to resolve any breaking changes

Author: matthiasgeihs

packages/seedless-onboarding-controller/src/utils.ts

@@ -30,6 +30,6 @@ export function decodeJWTToken(token: string): DecodedBaseJWTToken {
   const payload = parts[1];
   // Add padding if needed for base64 decoding
   const paddedPayload = payload + '='.repeat((4 - (payload.length % 4)) % 4);
-  const decoded = JSON.parse(Buffer.from(paddedPayload, 'base64').toString());
+  const decoded = JSON.parse(bytesToUtf8(base64ToBytes(paddedPayload)));
   return decoded as DecodedBaseJWTToken;
 }