Bump undici from 6.15.0 to 6.21.1

Bumps [undici](https://github.com/nodejs/undici) from 6.15.0 to 6.21.1.
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v6.15.0...v6.21.1)

---
updated-dependencies:
- dependency-name: undici
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]