submodule: bump openthread from 3f99b11 to 666a9bd (#637)

Bumps [openthread](https://github.com/openthread/openthread) from `3f99b11` to `666a9bd`.
- [Commits](openthread/openthread@3f99b11...666a9bd)

---
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>