Add support for Terraform v1.14

[wata727]

This PR adds support for Terraform v1.14.
Terraform v1.14 does not include any changes for TFLint, so this change is a documentation update only.

• Purity verification for builtin functions hashicorp/terraform#37001
  • Although it is somewhat related, this is mainly a mechanism to ensure consistency of function return values ​​when plan/apply are executed at different times, and is unrelated to TFLint, which is completed in a single evaluation.
  • This may be supported if codebase divergence is an issue or greater compatibility with Terraform is important.
• evaluate: return diagnostics instead of unknown for uninitialised locals and resources hashicorp/terraform#37663
  • In TFLint, many values ​​are treated as unknown, so it is expected behavior that uninitialized values ​​are treated as unknown.

By the way, action blocks were newly introduced in v1.14. There are now an increasing number of invalid references in events expressions such as before_create.
https://developer.hashicorp.com/terraform/language/v1.14.x/invoke-actions

TFLint has tried to handle references the same way as Terraform as much as possible, but considering the possibility that such meta-references may be referenced unexpectedly by plugins, it may be necessary to be more tolerant in how references are handled. In many cases, these references resolve to unknown values, but the important thing is that unintended errors can be prevented. For example, this can avoid issues like terraform-linters/tflint-ruleset-terraform#199.