Bump google/cloud-sdk in /dockerfiles/k8s-app-deployer

Bumps google/cloud-sdk from 588.0.0-slim to 591.0.0-slim.

---
updated-dependencies:
- dependency-name: google/cloud-sdk
  dependency-version: 591.0.0-slim
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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dockerfiles/k8s-app-deployer/Dockerfile

@@ -3,7 +3,7 @@
 
 # For deploying apps onto Kubernetes clusters (including GKE)
 
-FROM google/cloud-sdk:588.0.0-slim
+FROM google/cloud-sdk:591.0.0-slim
 
 # Install apache2-utils (for htpasswd to bcrypt passwords for the
 # local-user-authenticator) and jq.