submodule: bump openthread from a2d65a1 to 68a5ce3

Bumps [openthread](https://github.com/openthread/openthread) from `a2d65a1` to `68a5ce3`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@a2d65a1...68a5ce3)

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

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