submodule: bump openthread from 74f5ab5 to afc55a3 (#254)

Bumps [openthread](https://github.com/openthread/openthread) from `74f5ab5` to `afc55a3`.
- [Commits](openthread/openthread@74f5ab5...afc55a3)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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