https://wiki.centos.org/Download looks like they have 6.10, however can you instead try the certmgr test?
That “seems” like it should directly show what sort of trouble mono is going to run into with the certificates.
It seems like should be technically possible, but if you mean there’s no budget, time, etc., sorry about that. That software is a decade old now, out of support, at increasing risk, etc. – and with no recent backups…
A possible workaround would be to run a current Duplicati in a Docker container, if CentOS 6 can do that. This needs a manual fix for the same certificate expiration, but at least it has a solution. Yours doesn’t yet.
You could also try using Rclone as the Storage Type on the Destination screen - if you can get Rclone up.