submodule: bump openthread from f75e2bb to a0b9ac7 (#648)

Bumps [openthread](https://github.com/openthread/openthread) from `f75e2bb` to `a0b9ac7`.
- [Commits](openthread/openthread@f75e2bb...a0b9ac7)

---
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>