refactor: deprecate state upgraders

[vaishakdinesh]

related provider PR: cloudflare/terraform-provider-cloudflare#6686

What changed:
Moved state transformation logic from tf-migrate to the Terraform provider's built-in StateUpgraders.

Why:

• Simpler: tf-migrate now just transforms config files (HCL) - provider handles state
• More reliable: Provider's StateUpgraders run automatically during terraform apply
• Less code: Deleted ~2,300 lines of state transformation tests and logic

Key changes for 3 resources:

• dns_record: Removed state transform logic (59 KB → simpler)
• page_rule: Removed state transform logic
• load_balancer_pool: TransformState now returns state unchanged (no-op)

Test updates:

• ✅ Deleted state transform test cases
• ✅ Removed terraform.tfstate files from integration tests
• ✅ Config transformation tests still pass

Result: tf-migrate is now focused on config migration only. State migration happens automatically in the provider when users run terraform commands