Access list item with square bracket

[Yannic]

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Square bracket syntax is available since terraform v0.12, and this module requires >= v0.14.

Recommendation from TFLint:

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

  on main.tf line 80:
  80:   elasticache_member_clusters = module.this.enabled ? tolist(aws_elasticache_replication_group.default.0.member_clusters) : []

Reference: https://github.com/terraform-linters/tflint-ruleset-terraform/blob/v0.1.1/docs/rules/terraform_deprecated_index.md

what

• Describe high-level what changed as a result of these commits (i.e. in plain-english, what do these changes mean?)
• Use bullet points to be concise and to the point.

why

• Provide the justifications for the changes (e.g. business case).
• Describe why these changes were made (e.g. why do these commits fix the problem?)
• Use bullet points to be concise and to the point.

references

• Link to any supporting github issues or helpful documentation to add some context (e.g. stackoverflow).
• Use closes #123, if this PR closes a GitHub issue #123