Remove debian:11 CI build (#880)

Upstream Debian no longer care about this version:

"
...
E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/p/perl/libperl5.32_5.32.1-4%2bdeb11u5_amd64.deb  404  Not Found [IP: 151.101.22.132 80]
Fetched 302 MB in 7s (45.3 MB/s)
E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/g/gnupg2/gpgsm_2.2.27-2%2bdeb11u3_amd64.deb  404  Not Found [IP: 151.101.22.132 80]
E: Failed to fetch http://deb.debian.org/debian-security/pool/updates/main/g/glibc/libc6-dbg_2.31-13%2bdeb11u14_amd64.deb  404  Not Found [IP: 151.101.22.132 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
"

So neither shall we.

Signed-off-by: John Levon <john.levon@nutanix.com>

.github/workflows/pull_request.yml

diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index 9826d33..e834f94 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -47,19 +47,6 @@
               diffutils valgrind python3-pytest python3-flake8 which \
               meson ninja-build
           make pre-push
-  debian-11:
-    timeout-minutes: 10
-    runs-on: ubuntu-latest
-    container: debian:11
-    steps:
-      - uses: actions/checkout@v4
-      - name: pre-push
-        run: |
-          apt-get update
-          apt-get -y install build-essential cmake pkg-config libjson-c-dev \
-            libcmocka-dev clang clang-tools valgrind python3-pytest \
-            debianutils flake8 meson ninja-build
-          make pre-push
   debian-12:
     timeout-minutes: 10
     runs-on: ubuntu-22.04