submodule: bump openthread from cb1220d to 6bc3b4d

Bumps [openthread](https://github.com/openthread/openthread) from `cb1220d` to `6bc3b4d`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@cb1220d...6bc3b4d)

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

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