submodule: bump openthread from 50487a6 to 9f76d45 (#585)

Bumps [openthread](https://github.com/openthread/openthread) from `50487a6` to `9f76d45`.
- [Commits](openthread/openthread@50487a6...9f76d45)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>