submodule: bump openthread from a9805ed to 7886083

Bumps [openthread](https://github.com/openthread/openthread) from `a9805ed` to `7886083`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@a9805ed...7886083)

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

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