Bump third_party/ot-br-posix/repo from bd15f71 to 50ff1d1

Bumps [third_party/ot-br-posix/repo](https://github.com/openthread/ot-br-posix) from `bd15f71` to `50ff1d1`.
- [Release notes](https://github.com/openthread/ot-br-posix/releases)
- [Commits](openthread/ot-br-posix@bd15f71...50ff1d1)

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

Signed-off-by: dependabot[bot] <[email protected]>