submodule: bump openthread from e6a6b9f to 7bd3abd (#870)

Bumps [openthread](https://github.com/openthread/openthread) from `e6a6b9f` to `7bd3abd`.
- [Commits](openthread/openthread@e6a6b9f...7bd3abd)

---
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>