submodule: bump openthread from `864c4be` to `6d447f9` (#37)
submodule: bump openthread from `864c4be` to `6d447f9` (#37)

Bumps [openthread](https://github.com/openthread/openthread) from `864c4be` to `6d447f9`.
- [Commits](https://github.com/openthread/openthread/compare/864c4be9e5e8643440f5a1d5a277db175aa45b49...6d447f96da21e73b03927c27c1dfb99db7bc3e6a)

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

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-cc13x2-cc26x2,openthread/ot-cc13x2-cc26x2