fix: The type of the output `cloudwatch_log_group_name` should be a string instead of a list of strings (#1061)

NOTES: The output `cloudwatch_log_group_name` was incorrectly returning the log group name as a list of strings. As a workaround, people were using `module.eks_cluster.cloudwatch_log_group_name[0]` but that was totally inconsistent with output name. Those users can now use `module.eks_cluster.cloudwatch_log_group_name` directly.