submodule: bump third_party/openthread/repo from 473af53 to `2868c9…

…c` (#2630)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `473af53` to `2868c9c`.
- [Commits](openthread/openthread@473af53...2868c9c)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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