submodule: bump openthread from f07bcc2 to 9f6921c (#894)

Bumps [openthread](https://github.com/openthread/openthread) from `f07bcc2` to `9f6921c`.
- [Commits](openthread/openthread@f07bcc2...9f6921c)

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

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