submodule: bump third_party/openthread/repo from d31bcee to `9c2116…

…5` (#2689)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `d31bcee` to `9c21165`.
- [Commits](openthread/openthread@d31bcee...9c21165)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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