Added output for VPC flow log group name

[dms1981]

As part of #7607, we need to output the CloudWatch log group name for the VPC flow logs.

This PR adds a simple output so that the name can be used to populate an aws_cloudwatch_log_subscription_filter resource.

[github-actions]

Trivy Scan Success

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</details> #### `Checkov Scan` Success
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CTFLint Scan Success

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Setting default tflint config...
Running tflint --init...
Installing "terraform" plugin...
Installed "terraform" (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.5.0)
tflint will check the following folders:

Trivy Scan Success

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Trivy will check the following folders: