Bump third_party/ot-br-posix/repo from 6d70f5e to 51526d0

Bumps [third_party/ot-br-posix/repo](https://github.com/openthread/ot-br-posix) from `6d70f5e` to `51526d0`.
- [Release notes](https://github.com/openthread/ot-br-posix/releases)
- [Commits](openthread/ot-br-posix@6d70f5e...51526d0)

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

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

third_party/ot-br-posix/repo

@@ -0,0 +1 @@
+51526d026eaa7ff119d7a84ceec161f8a4da1380