ci: Drop Debian 11

Unfortunately, Debian 11 has just too old Rust which prevents us
from working on anything not prehistoric. There are couple of
issues:

1) It doesn't understand modern Cargo.toml syntax, which causes
   CI failures:

     https://gitlab.com/libvirt/libvirt-rust/-/merge_requests/64

2) We currently require bindgen-0.59.2 which doesn't work with
   LLVM-16 because of:

     rust-lang/rust-bindgen#2312

   The fix is contained in bindgen-0.62.0. While there is some
   intersect between versions available to Debian 11 version of
   Rust (0.65.1, 0.64.0, 0.63.0, 0.62.0) neither of them generate
   code that Debian 11 version of Rust (1.48.0) understands.
   The bindgen-0.62.0 release notes set the minimal required
   version of rust to 1.57.0 anyway:

     https://crates.io/crates/bindgen/0.62.0

We are stuck. And the only way out is just to drop Debian 11
support. Live long my old friend.

Signed-off-by: Michal Privoznik <[email protected]>

ci/gitlab/builds.yml

@@ -79,24 +79,6 @@ x86_64-centos-stream-9-local-env:
     NAME: centos-stream-9
 
 
-x86_64-debian-11-prebuilt-env:
-  extends: .native_build_job_prebuilt_env
-  needs:
-    - job: x86_64-debian-11-container
-      optional: true
-  allow_failure: false
-  variables:
-    NAME: debian-11
-
-x86_64-debian-11-local-env:
-  extends: .native_build_job_local_env
-  needs: []
-  allow_failure: false
-  variables:
-    IMAGE: docker.io/library/debian:11-slim
-    NAME: debian-11
-
-
 x86_64-debian-12-prebuilt-env:
   extends: .native_build_job_prebuilt_env
   needs:

ci/gitlab/containers.yml

@@ -28,13 +28,6 @@ x86_64-centos-stream-9-container:
     NAME: centos-stream-9
 
 
-x86_64-debian-11-container:
-  extends: .container_job
-  allow_failure: false
-  variables:
-    NAME: debian-11
-
-
 x86_64-debian-12-container:
   extends: .container_job
   allow_failure: false

ci/manifest.yml

@@ -26,8 +26,6 @@ targets:
 
   centos-stream-9: x86_64
 
-  debian-11: x86_64
-
   debian-12: x86_64
 
   debian-sid: