fix(sdk): update python protobuf package for security update (kubeflow#1066)

Author: Tomcli

sdk/python/requirements.in

@@ -1 +1,2 @@
 kfp>=1.8.10,<1.8.15
+protobuf>=3.20.1,<4

sdk/python/requirements.txt

@@ -79,8 +79,9 @@ oauthlib==3.1.0
     # via requests-oauthlib
 packaging==20.9
     # via google-api-core
-protobuf==3.15.2
+protobuf==3.20.3
     # via
+    #   -r sdk/python/requirements.in
     #   google-api-core
     #   googleapis-common-protos
     #   kfp
@@ -137,7 +138,6 @@ six==1.15.0
     #   jsonschema
     #   kfp-server-api
     #   kubernetes
-    #   protobuf
     #   python-dateutil
     #   websocket-client
 strip-hints==0.1.9