submodule: bump openthread from bf587dd to c73829f

Bumps [openthread](https://github.com/openthread/openthread) from `bf587dd` to `c73829f`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@bf587dd...c73829f)

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

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