submodule: bump openthread from `8b7d90f` to `7e438ca` (#186)
submodule: bump openthread from `8b7d90f` to `7e438ca` (#186)

Bumps [openthread](https://github.com/openthread/openthread) from `8b7d90f` to `7e438ca`.
- [Commits](https://github.com/openthread/openthread/compare/8b7d90f167a85801e2fd0cb2356b9f4a2d04153c...7e438caf304d8eb697c38288c00b5fc07e4b2d44)

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

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-samr21,openthread/ot-samr21