fix: TFlint warnings

[foxdalas]

Description

I have addressed the TFLint warnings by making the necessary adjustments to the code.

Motivation and Context

These changes resolve TFLint warnings related to the use of deprecated index access for list items and an unused variable declaration.

Warning: [Fixable] List items should be accessed using square brackets (terraform_deprecated_index)

  on main.tf line 5:
   5:   this_sg_id = var.create_sg ? concat(aws_security_group.this.*.id, aws_security_group.this_name_prefix.*.id, [""])[0] : var.security_group_id

Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.5.0/docs/rules/terraform_deprecated_index.md

Warning: [Fixable] List items should be accessed using square brackets (terraform_deprecated_index)

  on main.tf line 5:
   5:   this_sg_id = var.create_sg ? concat(aws_security_group.this.*.id, aws_security_group.this_name_prefix.*.id, [""])[0] : var.security_group_id

Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.5.0/docs/rules/terraform_deprecated_index.md

Warning: [Fixable] variable "auto_groups" is declared but not used (terraform_unused_declarations)

  on rules.tf line 192:
 192: variable "auto_groups" {

Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.5.0/docs/rules/terraform_unused_declarations.md

Breaking Changes

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

• I have executed pre-commit run -a on my pull request

[foxdalas]

@bryantbiggs @antonbabenko
One of you can watch this PR. Thank you

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