submodule: bump openthread from 4a41cb4 to 6955fde (#287)

Bumps [openthread](https://github.com/openthread/openthread) from `4a41cb4` to `6955fde`.
- [Commits](openthread/openthread@4a41cb4...6955fde)

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

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