submodule: bump openthread from 254043d to 516472c (#1147)

Bumps [openthread](https://github.com/openthread/openthread) from `254043d` to `516472c`.
- [Commits](openthread/openthread@254043d...516472c)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 516472c1b553f14b87ce8ead3a04723639b4d48e
  dependency-type: direct:production
...

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Author: dependabot[bot]