submodule: bump openthread from `1b72b2d` to `cf21d57` (#42)
submodule: bump openthread from `1b72b2d` to `cf21d57` (#42)

Bumps [openthread](https://github.com/openthread/openthread) from `1b72b2d` to `cf21d57`.
- [Commits](https://github.com/openthread/openthread/compare/1b72b2d82e3d91216e18af0a2c4f98fd788afdc5...cf21d5760a993ffedee7f6cd5983e03738b48de6)

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

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