Bump @vercel/build-utils from 13.3.0 to 13.21.0

Bumps [@vercel/build-utils](https://github.com/vercel/vercel/tree/HEAD/packages/now-build-utils) from 13.3.0 to 13.21.0.
- [Release notes](https://github.com/vercel/vercel/releases)
- [Commits](https://github.com/vercel/vercel/commits/@vercel/build-utils@13.21.0/packages/now-build-utils)

---
updated-dependencies:
- dependency-name: "@vercel/build-utils"
  dependency-version: 13.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]