Remove old integration resources

This merge request removes deprecated GitLab integration resources that were scheduled for removal in version 19.0. The changes eliminate the old gitlab_integration_* resources (for custom issue tracker, Harbor, and Jenkins integrations) and their associated documentation, examples, and test cases. These deprecated resources are being replaced by newer gitlab_project_integration_* resources that provide the same functionality with improved naming conventions. The code cleanup simplifies the codebase by removing duplicate resource definitions and backwards compatibility code that was no longer needed.

Part of #6512