docs(env): remove unnecessary comments and update MongoDB connection hint

- Remove redundant comments about required environment variables
- Clarify that the application cannot start without a database connection
- Remove outdated reference to dcli_scripts create_key

Author: fulleni

.env.example

@@ -3,12 +3,9 @@
 # The .env file is ignored by Git and should NOT be committed.
 
 # REQUIRED: The full connection string for your MongoDB instance.
-# The application cannot start without a database connection.
 # DATABASE_URL="mongodb://user:password@localhost:27017/flutter_news_app_api_server_full_source_code_db"
 
 # REQUIRED: A secure, randomly generated secret for signing JWTs.
-# The application cannot start without this.
-# Generate a secure key using: dart pub global run dcli_scripts create_key
 # JWT_SECRET_KEY="your-super-secret-and-long-jwt-key"
 
 # OPTIONAL: The duration for which a JWT is valid, in hours.