chore: Python 3.12 is the floor

netOrk no longer supports 3.10 or 3.11 anywhere. The actions run on the
runner's job container, catthehacker/ubuntu:act-24.04, whose python3 is
3.12; CI already tests on 3.12. Refs NAPALM/driver-standard#6.

README.md

@@ -60,7 +60,7 @@ with its CIS result and SBOM attached (NetOrk/netork#1390).
 ```
 
 The judge action reads `cis.toml` and `packages.allow` from the working directory by default.
-The runner needs Docker, Python 3.11 or newer, and `curl`; cosign is installed by the actions,
+The runner needs Docker, Python 3.12 or newer, and `curl`; cosign is installed by the actions,
 pinned and checked against its release checksum (`bin/install-cosign.sh`).
 
 ## Verifying an image yourself