submodule: bump openthread from `77d3af3` to `39e6172` (#22)
submodule: bump openthread from `77d3af3` to `39e6172` (#22)

Bumps [openthread](https://github.com/openthread/openthread) from `77d3af3` to `39e6172`.
- [Commits](https://github.com/openthread/openthread/compare/77d3af396c8bace8a1a6c9204c1a663ef751e9e6...39e6172304c4e5bdadf2fa6dbdc4d7a7f6987ebc)

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

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
openthread/ot-efr32,openthread/ot-efr32,openthread/ot-efr32