Add Computed to import attributes on gitlab_project

Closes #6710 (closed)

This change modifies two configuration fields in the GitLab project resource to prevent conflicts with a new pull mirror feature. The import_url and mirror fields are now marked as "computed", which means the system can automatically determine their values instead of requiring manual input. This prevents configuration conflicts when using the new gitlab_project_pull_mirror resource, allowing both the old and new mirror functionality to coexist without interfering with each other.