submodule: bump openthread from 4db6520 to 7d4a1f8 (#710)

Bumps [openthread](https://github.com/openthread/openthread) from `4db6520` to `7d4a1f8`.
- [Commits](openthread/openthread@4db6520...7d4a1f8)

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

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