submodule: bump openthread from 555454c to 7d488bc (#928)

Bumps [openthread](https://github.com/openthread/openthread) from `555454c` to `7d488bc`.
- [Commits](openthread/openthread@555454c...7d488bc)

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

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