submodule: bump openthread from a0ba929 to 99b3ed4 (#747)

Bumps [openthread](https://github.com/openthread/openthread) from `a0ba929` to `99b3ed4`.
- [Commits](openthread/openthread@a0ba929...99b3ed4)

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

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