Remove gitlab_project_job_token_scopes.project_id

This merge request removes a deprecated field from the GitLab project job token scopes configuration. Previously, users could specify a project using either project or project_id fields, but now only the project field is supported. The change simplifies the code by eliminating duplicate ways to do the same thing and removes the old project_id option that was scheduled for removal. All documentation and examples have been updated to reflect this change, and the import functionality now works consistently with the single project field. The migration is designed to be smooth - existing configurations using project_id can be automatically upgraded to use project instead without breaking functionality.

Fixes #6337 (closed)