Fix App Engine deploy and startup script false failures

[anth-volk]

Fixes #3514

Summary

• replace yes | gcloud app deploy with gcloud app deploy --quiet
• prevent false CI failures after successful App Engine deploys under set -o pipefail
• remove stale App Engine startup attempts to launch nonexistent policyengine_api/worker.py
• add a changelog fragment for the deploy/startup fixes

Testing

• bash -n .github/scripts/deploy_app_engine_version.sh
• bash -n gcp/policyengine_api/start.sh

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.00%. Comparing base (8a7c1df) to head (d07f6d2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3515   +/-   ##
=======================================
  Coverage   77.00%   77.00%           
=======================================
  Files          63       63           
  Lines        3418     3418           
  Branches      617      617           
=======================================
  Hits         2632     2632           
  Misses        612      612           
  Partials      174      174           

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