chore(bazel): update protobuf to v3.21.3 (#356)

* chore(bazel): update protobuf to v3.21.3

chore(bazel): update gax-java to 2.18.4

PiperOrigin-RevId: 463115700

Source-Link: googleapis/googleapis@52130a9

Source-Link: googleapis/googleapis-gen@6a4d9d9
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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>

packages/google-cloud-securitycenter/google/cloud/securitycenter_v1beta1/services/security_center/async_client.py

@@ -1491,9 +1491,6 @@ async def sample_run_asset_discovery():
 
                       }
 
-                   The JSON representation for Empty is empty JSON
-                   object {}.
-
         """
         # Create or coerce a protobuf request object.
         # Quick check: If we got a request object, we should *not* have

packages/google-cloud-securitycenter/google/cloud/securitycenter_v1beta1/services/security_center/client.py

@@ -1703,9 +1703,6 @@ def sample_run_asset_discovery():
 
                       }
 
-                   The JSON representation for Empty is empty JSON
-                   object {}.
-
         """
         # Create or coerce a protobuf request object.
         # Quick check: If we got a request object, we should *not* have