submodule: bump openthread from cf733a3 to 3d975f7 (#772)

Bumps [openthread](https://github.com/openthread/openthread) from `cf733a3` to `3d975f7`.
- [Commits](openthread/openthread@cf733a3...3d975f7)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 3d975f7f9daea39f5fa45566c52381ede05d4e85
  dependency-type: direct:production
...

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