Fix AWS Provider v5+ compatibility for EIP resource

Update deprecated `vpc = true` to `domain = "vpc"` in aws_eip resource.

The `vpc` argument was deprecated in AWS Provider v5.0 and removed completely.
The new syntax uses `domain = "vpc"` instead.

This fixes test failures where Terraform validation fails with:
"Error: Unsupported argument - An argument named 'vpc' is not expected here"

Fixes the same issue that caused PR #225 tests to fail.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: aknysh

examples/existing-ips/main.tf

@@ -15,7 +15,7 @@ module "vpc" {
 resource "aws_eip" "nat_ips" {
   count = length(var.availability_zones)
 
-  vpc = true
+  domain = "vpc"
 
   depends_on = [
     module.vpc