submodule: bump openthread from 666a9bd to ee74513 (#762)

Bumps [openthread](https://github.com/openthread/openthread) from `666a9bd` to `ee74513`.
- [Commits](openthread/openthread@666a9bd...ee74513)

---
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>