[docker] upgrade gcloud version to 293.0.0 (GoogleCloudPlatform#3859)

Author: busunkim96

.kokoro/docker/Dockerfile

@@ -150,7 +150,7 @@ RUN rm /tmp/get-pip.py
 RUN pip install --no-cache-dir virtualenv
 
 # Setup Cloud SDK
-ENV CLOUD_SDK_VERSION 272.0.0
+ENV CLOUD_SDK_VERSION 293.0.0
 # Use system python for cloud sdk.
 ENV CLOUDSDK_PYTHON python3.6
 RUN wget https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud-sdk-$CLOUD_SDK_VERSION-linux-x86_64.tar.gz