submodule: bump openthread from baf13fc to 8a19434 (#984)

Bumps [openthread](https://github.com/openthread/openthread) from `baf13fc` to `8a19434`.
- [Commits](openthread/openthread@baf13fc...8a19434)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 8a19434b8ae56ed5ffbc931d3f1aa212823633c6
  dependency-type: direct:production
...

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