submodule: bump openthread from a54f4c4 to 70bad1d

Bumps [openthread](https://github.com/openthread/openthread) from `a54f4c4` to `70bad1d`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@a54f4c4...70bad1d)

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

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