handle invalid mirrors
Some mirrors can be invalid (expired domains) such as http://mirror.connorfreebairn.rocks/ubuntu/ (as of 09/06/2025). These should be automatically discarded from the candidate list during nala fetch
Some mirrors can be invalid (expired domains) such as http://mirror.connorfreebairn.rocks/ubuntu/ (as of 09/06/2025). These should be automatically discarded from the candidate list during nala fetch