terraform_required_version: fix regression with multiple blocks

[bendrucker]

Fixes a regression where multiple terraform {} blocks results in a false positive issue being emitted. https://github.com/terraform-linters/tflint/pull/1433/files#diff-0ec954b5c7913111444d96417f3325c4a2f5d91507770a89022bcbd4b05319cb, which rewrote chunks of the terraformrules package to eliminate dependencies on Terraform internals, introduced this regression. The new code, which operates on the generic HCL structure rather than the parsed Terraform internal representation has a simple logic error:

for _, block := range body.Blocks {
	if _, exists := block.Body.Attributes["required_version"]; !exists {

This processes every terraform block and emits and error if required_version is unset. Instead, the rule should process all the blocks and emit a single error if none has the attribute set.

Fixes #1451