Bump third_party/ot-br-posix/repo from b0bbad3 to f6a0a81 (#28774)

Bumps [third_party/ot-br-posix/repo](https://github.com/openthread/ot-br-posix) from `b0bbad3` to `f6a0a81`.
- [Release notes](https://github.com/openthread/ot-br-posix/releases)
- [Commits](openthread/ot-br-posix@b0bbad3...f6a0a81)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>