go/flutter-cirrus#create-a-new-key

Update existing repos(flutter/flutter, flutter/engine, flutter/plugins, flutter/packages, flutter/assets-for-api-docs) with the new key
Encrypt the above key in cirrus settings for flutter org
Update .cirrus.yaml with the new encrypted value

Update flutter/plugins firebase test lab key with the new key
Encrypt the key in flutter/plugins repo (this needs write access)
Update GCLOUD_FIREBASE_TESTLAB_KEY in .cirrus.yaml with the new encrypted value. Note this should be different from the above GCP key though we are encrypting the same value (https://cirrus-ci.org/guide/writing-tasks/#encrypted-variables)

Please add a calendar reminder to flutter's infra calendar 1 week before the cirrus key expires, so that the gardener can renew the key before it expires.