fix(deps): update dependency @google-cloud/firestore to ^0.21.0#1100

fhinkel merged 2 commits intomasterGoogleCloudPlatform/nodejs-docs-samples:masterfrom

renovate/google-cloud-firestore-0.xGoogleCloudPlatform/nodejs-docs-samples:renovate/google-cloud-firestore-0.xCopy head branch name to clipboard