submodule: bump openthread from 35424b3 to 266a97b (#650)

Bumps [openthread](https://github.com/openthread/openthread) from `35424b3` to `266a97b`.
- [Commits](openthread/openthread@35424b3...266a97b)

---
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>