submodule: bump openthread from f12dfaf to 9f6921c

Bumps [openthread](https://github.com/openthread/openthread) from `f12dfaf` to `9f6921c`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@f12dfaf...9f6921c)

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

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