The error message tells you exactly what's causing the problem: "Unable to delete 'PROD_ETL' because it is being referenced by 'BI - Update Definition Change Tracking'.
In the ADF UI, navigate to the pipeline "BI - Update Definition Change Tracking." and carefully inspect every single activity within this pipeline looking for any reference to the deleted linked service "PROD_ETL."
Remove all the references then go to the "Manage" hub, Git configuration, and commit the changes. Now try publishing.