## Because
- Node 16 added `base64url` support to `Buffer`. The `base64url` package repeats work that Node does.
- The parent task FXA-13426 removes old dependencies from the monorepo.
## This pull request
- Removes `base64url` from the root `package.json` and from `yarn.lock`.
- Replaces each call with `Buffer.from(value, 'base64url')` or `buffer.toString('base64url')`.
- Drops the `base64url` re-export from the crypto-relier deriver barrel file.
- Keeps a manual `toBase64Url` helper in `scoped-keys.ts`. fxa-content-server bundles that file for the browser. Its `buffer` polyfill (6.0.3) throws "Unknown encoding: base64url".
- Updates `lib/push.js`, `lib/pushbox/index.ts`, and five `test/remote/*.in.spec.ts` specs in fxa-auth-server.
## Issue that this pull request solves
Closes: https://mozilla-hub.atlassian.net/browse/FXA-13918