Add state mover for gitlab_group_deploy_token

Summary

This change adds the ability to migrate existing GitLab deploy tokens from an older, deprecated resource type (gitlab_deploy_token) to a newer, more specific one (gitlab_group_deploy_token) without destroying and recreating them. Previously, users who wanted to switch to the new resource would lose their token value (which is only shown once at creation time). Now, Terraform can simply reassign the existing token to the new resource type, preserving all its data. This migration requires Terraform version 1.8 or later. A test was also added to verify the migration works correctly end-to-end, including confirming the token value is preserved after the move.

Fixes #6833 (closed)