submodule: bump third_party/openthread/repo from f621312 to `f75e2b…

…b` (#2162)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `f621312` to `f75e2bb`.
- [Commits](openthread/openthread@f621312...f75e2bb)

---
updated-dependencies:
- dependency-name: third_party/openthread/repo
  dependency-type: direct:production
...

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