(docs deps): Bump undici from 6.19.8 to 6.21.1 in the npm_and_yarn group

Bumps the npm_and_yarn group with 1 update: [undici](https://github.com/nodejs/undici).


Updates `undici` from 6.19.8 to 6.21.1
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v6.19.8...v6.21.1)

---
updated-dependencies:
- dependency-name: undici
  dependency-type: indirect
  dependency-group: npm_and_yarn
...

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

Author: dependabot[bot]