submodule: bump openthread from b0e7b14 to 31adbd0

Bumps [openthread](https://github.com/openthread/openthread) from `b0e7b14` to `31adbd0`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b0e7b14...31adbd0)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 31adbd0edc0a62557b12c8f2173b89abc14807cc
  dependency-type: direct:production
...

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Author: dependabot[bot]