submodule: bump third_party/openthread/repo from df12824 to `f31a58…

…3` (#2037)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `df12824` to `f31a583`.
- [Commits](openthread/openthread@df12824...f31a583)

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

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