submodule: bump openthread from 694528a to 31df636

Bumps [openthread](https://github.com/openthread/openthread) from `694528a` to `31df636`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@694528a...31df636)

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

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