fix: Existing linting issues (#290)

* `lookup` call with two arguments is deprecated since Terraform v0.7 [1].
* Full splat operator is available since Terraform v0.12 [2].

[1] https://developer.hashicorp.com/terraform/language/functions/lookup
[2] https://www.hashicorp.com/blog/terraform-0-12-generalized-splat-operator

Author: adamantike