submodule: bump openthread from dc6bf36 to 75694d2 (#636)

Bumps [openthread](https://github.com/openthread/openthread) from `dc6bf36` to `75694d2`.
- [Commits](openthread/openthread@dc6bf36...75694d2)

---
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>