enable Cloud Messaging API (Legacy) is not allowd anymore for sending notifications

Google has officially deprecated the legacy Cloud Messaging API in favor of the newer, more secure FCM HTTP v1 API, which uses OAuth 2.0 access tokens. The legacy API was shut down in July 2024, so you can no longer enable or use it for new projects.
If you are migrating an existing application, you can follow these general steps to start using the current API:
Access your Firebase project: Go to the Firebase console and select your project.
Navigate to Cloud Messaging settings: Click the gear icon next to "Project Overview," select Project settings, and then click the Cloud Messaging tab.
Enable the Firebase Cloud Messaging API (V1):
Find the Firebase Cloud Messaging API (V1) section.
If it's not already enabled, click the "Manage API in Google Cloud Console" button to enable it.
Create a service account key:
In the Service accounts tab of your project settings, click Generate new private key.
Download the JSON file, which contains your OAuth 2.0 credentials.
Update your server code: Use a client library for the Firebase Admin SDK or manually update your code to use the HTTP v1 API with the new service account credentials.
For further guidance, consult the official Firebase documentation for migrating from the legacy APIs to HTTP v1.

Locate the documentation covering Cloud Messaging notifications and identify any instructions that still depend on the deprecated legacy API. Compare them with Firebase's current FCM HTTP v1 migration guidance; done means the obsolete instructions are updated and the relevant Firebase links and setup steps are accurate.