[Installers] New signing cert requirements for publishing rpm and deb installers to PMC

Requirement details at https://eng.ms/docs/cloud-ai-platform/azure-core/azure-cloud-native-and-management-platform/azcorelinux/pmc-package-ingestion/pmc-onboardingreference/publish/signing_key_transition

This is somewhat urgent as it blocks us from publishing to Debian 13, which recently GA'd. 