submodule: bump openthread from d42343c to 51353c4 (#1140)

Bumps [openthread](https://github.com/openthread/openthread) from `d42343c` to `51353c4`.
- [Commits](openthread/openthread@d42343c...51353c4)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 51353c41d544d37ead7a8ff67936c61cdac289e2
  dependency-type: direct:production
...

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Author: dependabot[bot]