Add state mover for gitlab_project_integration_harbor

This code change adds support for migrating from an old GitLab Harbor integration resource to a new one. The main improvement is that users can now seamlessly upgrade their Terraform configurations from the deprecated gitlab_integration_harbor resource to the newer gitlab_project_integration_harbor resource without losing their existing settings.

The change implements a state migration feature that automatically transfers all the configuration data (like Harbor server URL, username, password, and project settings) from the old resource format to the new one when users update their Terraform code. This migration requires Terraform version 1.8 or later to work properly.

The code also includes comprehensive tests to ensure the migration works correctly - it verifies that the old resource can be created, successfully moved to the new resource type, and continues to function properly after the migration. This allows users to modernize their infrastructure code without having to manually recreate their Harbor integrations.

Part of #6717