submodule: bump openthread from a0b9ac7 to 00076af

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

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

Signed-off-by: dependabot[bot] <[email protected]>