submodule: bump openthread from 7f81453 to c13fcc6 (#862)

Bumps [openthread](https://github.com/openthread/openthread) from `7f81453` to `c13fcc6`.
- [Commits](openthread/openthread@7f81453...c13fcc6)

---
updated-dependencies:
- dependency-name: openthread
  dependency-type: direct:production
...

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