docs(ecs): remove restriction regarding environment files against Fargate (#27081)

Currently, CDK document includes the following description.
https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ecs-readme.html#environment-variables
> Support for environment files is restricted to the EC2 launch type for files hosted on S3. 

However, this is out of date. Fargate has been [supported environment files](https://aws.amazon.com/jp/blogs/containers/latest-updates-to-aws-fargate-for-amazon-ecs/), and this restriction on CDK had been already removed by #11820. 

This PR removes the out of date description from document.

Closes #18226

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: tam0ri

packages/aws-cdk-lib/aws-ecs/README.md

@@ -490,8 +490,7 @@ newContainer.addSecret('API_KEY', ecs.Secret.fromSecretsManager(secret));
 newContainer.addSecret('DB_PASSWORD', ecs.Secret.fromSecretsManager(secret, 'password'));
 ```
 
-The task execution role is automatically granted read permissions on the secrets/parameters. Support for environment
-files is restricted to the EC2 launch type for files hosted on S3. Further details provided in the AWS documentation
+The task execution role is automatically granted read permissions on the secrets/parameters. Further details provided in the AWS documentation
 about [specifying environment variables](https://docs.aws.amazon.com/AmazonECS/latest/developerguide/taskdef-envfiles.html).
 
 ### Linux parameters