submodule: bump openthread from a0b9ac7 to 00076af (#775)

Bumps [openthread](https://github.com/openthread/openthread) from `a0b9ac7` to `00076af`.
- [Commits](openthread/openthread@a0b9ac7...00076af)

---
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>