submodule: bump openthread from 1e82c4e to 6751122 (#589)

Bumps [openthread](https://github.com/openthread/openthread) from `1e82c4e` to `6751122`.
- [Commits](openthread/openthread@1e82c4e...6751122)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>