submodule: bump openthread from e6a6b9f to 2868c9c

Bumps [openthread](https://github.com/openthread/openthread) from `e6a6b9f` to `2868c9c`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@e6a6b9f...2868c9c)

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

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