docs: Add note for next breaking change to remove `load_balancers` and `target_group_arns` from use

[bryantbiggs]

Description

• Add note for next breaking change to remove load_balancers and target_group_arns from use

Motivation and Context

• With the addition of aws_autoscaling_traffic_source_attachment in #248, this has introduced a second point of control for load balancer attachment which started showing up in diffs on the autoscaling group resource. In #252 we have suppressed this diff by ignoring all changes to load_balancers and target_group_arns used directly on the autoscaling group resource. In hindsight, this was not an ideal route for users. Going forward, at the next breaking change we should remove the use of load_balancers and target_group_arns and strictly use the traffic_source_* arguments. The aws_autoscaling_traffic_source_attachment is more flexible and supports more resources than what the ASG resource does directly today. You can think of the similar lifecycle that security groups and security group rules have gone through (first rules were set on the group resource, later a separate resource was added for rules only, and then later a 3rd more explicit rule resource was added - all to differentiate between the different lifecycles of the resources and their behaviors)

Breaking Changes

• No

How Has This Been Tested?

• [ ] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [ ] I have tested and validated these changes using one or more of the provided examples/* projects
• [x] I have executed pre-commit run -a on my pull request

[github-actions[bot]]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.

[antonbabenko]

This PR is included in version 7.4.0 :tada: