submodule: bump third_party/openthread/repo from a50d1a7 to 9842f4b (#3123)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `a50d1a7` to `9842f4b`.
- [Commits](openthread/openthread@a50d1a7...9842f4b)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-version: 9842f4bd25cc291c7b871a2b6342b8aeee4b13ca
  dependency-type: direct:production
...

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Author: dependabot[bot]