ci: bump debian-11 job to debian-12

Debian 11 just recently went out of its LTS period, and is unmaintained
by the project (there is "Extended LTS", but it is a paid service
provided by a third party).

The point of the debian-11 job was to cover older releases in the LTS
state, per ac112fd4f0 (Add additional CI jobs to avoid accidental
breakage, 2024-10-31). Bumping to debian-12 will cover us there for the
next 2 years.

Signed-off-by: Jeff King <peff@peff.net>

.github/workflows/main.yml

diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index cf341d7..e5a8ef5 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -411,9 +411,9 @@
         # 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}}

.gitlab-ci.yml

diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 1a8e909..d321eb0 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -71,9 +71,9 @@
       # 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
   artifacts:
     paths:
       - t/failed-test-artifacts