submodule: bump third_party/openthread/repo from `b4c76b3` to `19cd73d` (#1096)

Bumps [third_party/openthread/repo](https://github.com/openthread/openthread) from `b4c76b3` to `19cd73d`.
- [Commits](https://github.com/openthread/openthread/compare/b4c76b3e170a729f9bdb57ddaa6d05b73e541ce2...19cd73d69121c33d8d65cb0c4524cc5aa9084bd0)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
diff --git a/third_party/openthread/repo b/third_party/openthread/repo
index b4c76b3..19cd73d 160000
--- a/third_party/openthread/repo
+++ b/third_party/openthread/repo
@@ -1 +1 @@
-Subproject commit b4c76b3e170a729f9bdb57ddaa6d05b73e541ce2
+Subproject commit 19cd73d69121c33d8d65cb0c4524cc5aa9084bd0