Bump third_party/ot-br-posix/repo from ff2f24c to 1576ade

Bumps [third_party/ot-br-posix/repo](https://github.com/openthread/ot-br-posix) from `ff2f24c` to `1576ade`.
- [Release notes](https://github.com/openthread/ot-br-posix/releases)
- [Commits](openthread/ot-br-posix@ff2f24c...1576ade)

---
updated-dependencies:
- dependency-name: third_party/ot-br-posix/repo
  dependency-type: direct:production
...

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