submodule: bump openthread from 3d975f7 to 181405e

Bumps [openthread](https://github.com/openthread/openthread) from `3d975f7` to `181405e`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@3d975f7...181405e)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 0f4fbabc13815192f299e4dfc254256982b861d5
  dependency-type: direct:production
...

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Author: dependabot[bot]