submodule: bump openthread from 43cb7a0 to a234add

Bumps [openthread](https://github.com/openthread/openthread) from `43cb7a0` to `a234add`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@43cb7a0...a234add)

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

Signed-off-by: dependabot[bot] <support@github.com>