Bazel: Load java_proto_library from protobuf

The code-owners plugin still loads java_proto_library from rules_java,
which is deprecated and causes Bazel warnings during the build.

Switch the load statement to:
@protobuf//bazel:java_proto_library.bzl

This matches current Gerrit usage and removes the deprecation warnings
for owners_metadata_java_proto.

Change-Id: I9207bc6829e5947d2441487cf62b9d54e311f105

proto/BUILD

diff --git a/proto/BUILD b/proto/BUILD
index ed0f86f..59bba8a 100644
--- a/proto/BUILD
+++ b/proto/BUILD
@@ -1,5 +1,5 @@
+load("@protobuf//bazel:java_proto_library.bzl", "java_proto_library")
 load("@rules_proto//proto:defs.bzl", "proto_library")
-load("@rules_java//java:defs.bzl", "java_proto_library")
 
 proto_library(
     name = "owners_metadata_proto",