submodule: bump openthread from 59b87d6 to 9f6921c

Bumps [openthread](https://github.com/openthread/openthread) from `59b87d6` to `9f6921c`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@59b87d6...9f6921c)

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

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