Bump undici in /samples/platformatic/app

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

Updates `undici` from 5.28.4 to 5.29.0
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v5.28.4...v5.29.0)

Updates `undici` from 6.19.8 to 6.21.3
- [Release notes](https://github.com/nodejs/undici/releases)
- [Commits](nodejs/undici@v5.28.4...v5.29.0)

---
updated-dependencies:
- dependency-name: undici
  dependency-version: 5.29.0
  dependency-type: indirect
- dependency-name: undici
  dependency-version: 6.21.3
  dependency-type: indirect
...

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Author: dependabot[bot]