submodule: bump openthread from 830572f to 666a9bd

Bumps [openthread](https://github.com/openthread/openthread) from `830572f` to `666a9bd`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@830572f...666a9bd)

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

Signed-off-by: dependabot[bot] <[email protected]>