submodule: bump openthread from b0e7b14 to 87a919f

Bumps [openthread](https://github.com/openthread/openthread) from `b0e7b14` to `87a919f`.
- [Release notes](https://github.com/openthread/openthread/releases)
- [Commits](openthread/openthread@b0e7b14...87a919f)

---
updated-dependencies:
- dependency-name: openthread
  dependency-version: 87a919fd0a96e779a17e5cdd7bafc6c5b9e744e2
  dependency-type: direct:production
...

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Author: dependabot[bot]