submodule: bump openthread from 8a19434 to b945928 (#985)

Bumps [openthread](https://github.com/openthread/openthread) from `8a19434` to `b945928`.
- [Commits](openthread/openthread@8a19434...b945928)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: b945928d722177cd9caeab2e1025499628c101ef
  dependency-type: direct:production
...

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Author: dependabot[bot]