functions で `functions.config()` API を使用している箇所をマイグレートする

[hakatashi]

Firebase Functions で活用している functions.config() は既にdeprecatedで、2026年3月に停止するとのこと

! DEPRECATION NOTICE: Action required to deploy after March 2026

  functions.config() API is deprecated.
  Cloud Runtime Configuration API, the Google Cloud service used to store function configuration data, will be shut down in March 2026. As a result, you must migrate away from using functions.config() to continue deploying your functions after March 2026.

  What this means for you:

  - The Firebase CLI commands for managing this configuration (functions:config:set, get, unset, clone, and export) are deprecated. These commands will no longer work after March 2026.
  - firebase deploy command will fail for functions that use the legacy functions.config() API after March 2026.     

  Existing deployments will continue to work with their current configuration.

  See your migration options at: https://firebase.google.com/docs/functions/config-env#migrate-to-dotenv