Microsoft information:
https://support.microsoft.com/en-us/topic/windows-secure-boot-certificate-expiration-and-ca-updates-7ff40d33-95dc-4c3c-8725-a9b95457578e
https://support.microsoft.com/en-us/topic/frequently-asked-questions-about-the-secure-boot-update-process-b34bf675-b03a-4d34-b689-98ec117c7818
Some certificates available here:
https://learn.microsoft.com/en-au/windows-hardware/manufacture/desktop/windows-secure-boot-key-creation-and-management-guidance?view=windows-11#14-signature-databases-db-and-dbx
But a BIOS update might be required. Good luck with that for old PCs
DELL information:
https://www.dell.com/support/kbdoc/en-US/000347876/microsoft-2011-secure-boot-certificate-expiration
HP information:
https://support.hp.com/us-en/document/ish_13070353-13070429-16
