fix: Argument target_group_arn should not be defined if listener type is forward and traffic needs to be routed to multiple target groups

[romogo17]

Description

Fixes #299

Motivation and Context

Fixes bug that currently makes weighted target groups not to work as expected, documented more in-depth in the issue above.

Breaking Changes

No

How Has This Been Tested?

I have tested this module with a listener that is using weighted target groups, like the one provided in the examples

terraform-aws-alb/examples/complete-alb/main.tf

Lines 82 to 98 in cb8e43d

	{
	port = 81
	protocol = "HTTP"
	action_type = "forward"
	forward = {
	target_groups = [
	{
	target_group_index = 0
	weight = 100
	},
	{
	target_group_index = 1
	weight = 0
	}
	]
	}
	},

Now, when more than 1 target group is defined, the default target_group_arn is removed, and the target groups are configured as expected in the forward block

      ~ default_action {
          - target_group_arn = "arn:aws:elasticloadbalancing:us-east-1:012345678912:targetgroup/MY-TARGET-GROUP-1/x0x0x0x0x0x0x" -> null
            # (2 unchanged attributes hidden)

          + forward {
              + stickiness {
                  + duration = 3600
                  + enabled  = true
                }

              + target_group {
                  + arn    = "arn:aws:elasticloadbalancing:us-east-1:012345678912:targetgroup/MY-TARGET-GROUP-1/x0x0x0x0x0x0x"
                  + weight = 100
                }
              + target_group {
                  + arn    = "arn:aws:elasticloadbalancing:us-east-1:012345678912:targetgroup/MY-TARGET-GROUP-2/x0x0x0x0x0x0x"
                  + weight = 0
                }
            }
        }

Further apply's don't keep on trying to configure the forward section indefinitely.

[romogo17]

@antonbabenko, @bryantbiggs, any chance you can help me with feedback for this fix? Thanks beforehand!

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