submodule: bump openthread from 148fa47 to 8b906eb

Bumps [openthread](https://github.com/openthread/openthread) from `148fa47` to `8b906eb`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@148fa47...8b906eb)

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

Signed-off-by: dependabot[bot] <support@github.com>