submodule: bump third_party/openthread/repo from f75e2bb to `a0b9ac…

…7` (#2163)

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

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>