resource/lb_listener_rule: Fix action.forward.target_group minimum

[don-code]

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This change mirrors the one made in #15112, but for nondefault rule attachments on load balancers. The particular issue fixed is as follows when attempting to create a forwarding rule with only one target group:

resource "aws_lb_listener_rule" "default" {
  listener_arn = aws_alb_listener.listener.arn
  priority = 50000

  action {
    type = "forward"

    forward {
      target_group {
        arn = aws_alb_target_group.target_group.arn
      }
    }
  }
}

Error: List shorter than MinItems

  on main.tf line 354, in resource "aws_lb_listener_rule" "default":
 354:       target_group {

Attribute supports 2 item minimum, config has 1 declared

Closes #18072

[github-actions]

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[zhelding]

Pull request #21306 has significantly refactored the AWS Provider codebase. As a result, most PRs opened prior to the refactor now have merge conflicts that must be resolved before proceeding.

Specifically, PR #21306 relocated the code for all AWS resources and data sources from a single aws directory to a large number of separate directories in internal/service, each corresponding to a particular AWS service. This separation of code has also allowed for us to simplify the names of underlying functions -- while still avoiding namespace collisions.

We recognize that many pull requests have been open for some time without yet being addressed by our maintainers. Therefore, we want to make it clear that resolving these conflicts in no way affects the prioritization of a particular pull request. Once a pull request has been prioritized for review, the necessary changes will be made by a maintainer -- either directly or in collaboration with the pull request author.

For a more complete description of this refactor, including examples of how old filepaths and function names correspond to their new counterparts: please refer to issue #20000.

For a quick guide on how to amend your pull request to resolve the merge conflicts resulting from this refactor and bring it in line with our new code patterns: please refer to our Service Package Refactor Pull Request Guide.