submodule: bump openthread from d63da4f to 39fcc5e (#699)

Bumps [openthread](https://github.com/openthread/openthread) from `d63da4f` to `39fcc5e`.
- [Commits](openthread/openthread@d63da4f...39fcc5e)

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

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