ci: move the Debian job to a supported image

Debian 11 reached the end of LTS support on August 31, 2026. Its
security repository metadata has expired, so the job fails during
apt-get update before checking out the source.

Run the existing job on Debian 12, whose LTS support lasts through
June 30, 2028. The Debian dependency setup already accepts its image
and job name.

Signed-off-by: Taylor Blau <ttaylorr@openai.com>

Author: ttaylorr-oai

.github/workflows/main.yml

@@ -411,9 +411,9 @@ jobs:
         # A RHEL 8 compatible distro.  Supported until 2029-05-31.
         - jobname: almalinux-8
           image: almalinux:8
-        # Supported until 2026-08-31.
-        - jobname: debian-11
-          image: debian:11
+        # Supported until 2028-06-30.
+        - jobname: debian-12
+          image: debian:12
     env:
       jobname: ${{matrix.vector.jobname}}
       CC: ${{matrix.vector.cc}}