Bump third_party/openthread/repo from 434ea2c to 809cdc4 (#27595)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `434ea2c` to `809cdc4`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@434ea2c...809cdc4)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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