Bump undici in /xref-helper (#707)

Bumps  and [undici](https://github.com/nodejs/undici). These dependencies needed to be updated together.

Updates `undici` from 7.28.0 to 7.29.0
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v7.28.0...v7.29.0)

Updates `undici` from 6.27.0 to 6.28.0
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v7.28.0...v7.29.0)

---
updated-dependencies:
- dependency-name: undici
  dependency-version: 7.29.0
  dependency-type: indirect
- dependency-name: undici
  dependency-version: 6.28.0
  dependency-type: indirect
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]