Add Regression Handling to Certificate Matrix

Sometimes OSes that previously passed/achieved a certain certification level regress - for example, a few weeks ago Debian 14 (Forky) pushed a new QEMU version that interacted badly with its default kernel and prevented the VM from coming up in a dispatch run.

Handle revoking a certification in the [README.md certification matrix](https://github.com/amd-aliem/sev-certify#certification-matrix). The table should show the new level, an ❌ , and potentially a small note that it's a regression.

The GH workflow could be updated to NOT close a previous passing certification in this case, and/or attach some sort of 'regression` label.