Update protobuf dependency in ads template

Author: vchudnov-g

gapic/ads-templates/setup.py.j2

@@ -34,6 +34,7 @@ dependencies = [
     "googleapis-common-protos >= 1.53.0",
     "grpcio >= 1.10.0",
     "proto-plus >= 1.22.3, <2.0.0",
+    "protobuf >= 4.25.8, <5.0.0dev",
     {% if api.requires_package(('google', 'iam', 'v1')) %}
     "grpc-google-iam-v1",
     {% endif %}