eventbridge-cloudwatch-terraform: Update AWS Provider to v6

[kakakakakku]

Issue #, if available:
N/A

Description of changes:
Hi😀 Thanks for the useful patterns!

To keep this pattern maintainable, I updated the AWS Provider to v6 and fixed the CloudWatch Logs resource policy and README that were broken due to the outdated provider version.

Check

terraform apply completed successfully and works good.

$ aws events put-events --entries file://event.json --region us-east-1
{
    "FailedEntryCount": 0,
    "Entries": [
        {
            "EventId": "a4225bec-15eb-d539-4a8f-8e383e0d6237"
        }
    ]
}

Thank you😀

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[kakakakakku]

note: Removed an incorrect reference to "AWS SAM template" and "IAM permissions" since this is a Terraform pattern and no IAM resources are defined.

[kakakakakku]

ref: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/guides/version-3-upgrade.html#resource-aws_cloudwatch_log_group