submodule: bump openthread from bb5e9b7 to 7673194 (#796)

Bumps [openthread](https://github.com/openthread/openthread) from `bb5e9b7` to `7673194`.
- [Commits](openthread/openthread@bb5e9b7...7673194)

---
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>