Stop supporting debian11 (#2866)

Debian 11 is past its EOL, and even past LTS.
https://www.debian.org/News/2026/20260831

Debian has stopped publishing older dependencies required by version 11,
and we can no longer build it.

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 1952461..db6db3f 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -336,6 +336,7 @@
 # IMAGE_HASHES dict.
 EOL_PLATFORMS = frozenset(
     [
+        "debian11",
         "ubuntu1604",
         "ubuntu1804",
         "ubuntu2004",

buildkite/docker/build.sh

diff --git a/buildkite/docker/build.sh b/buildkite/docker/build.sh
index 916dbf1..2af06fa 100755
--- a/buildkite/docker/build.sh
+++ b/buildkite/docker/build.sh
@@ -62,7 +62,6 @@
 pids=()
 docker_build -f rockylinux8/Dockerfile  --builder mp-builder --load --platform=linux/amd64,linux/arm64 --target rockylinux8 -t "gcr.io/$PREFIX/rockylinux8"  rockylinux8 & pids+=($!)
 docker_build -f debian10/Dockerfile   --target debian10-java11   -t "gcr.io/$PREFIX/debian10-java11" debian10 & pids+=($!)
-docker_build -f debian11/Dockerfile   --target debian11-java17   -t "gcr.io/$PREFIX/debian11-java17" debian11 & pids+=($!)
 docker_build -f debian12/Dockerfile   --builder mp-builder --load --platform=linux/amd64,linux/arm64 --target debian12-java17   -t "gcr.io/$PREFIX/debian12" debian12 & pids+=($!)
 docker_build -f debian13/Dockerfile   --builder mp-builder --load --platform=linux/amd64,linux/arm64 --target debian13-java21   -t "gcr.io/$PREFIX/debian13" debian13 & pids+=($!)
 docker_build -f ubuntu2204/Dockerfile   --builder mp-builder --load --platform=linux/amd64,linux/arm64 --target ubuntu2204-java17 -t "gcr.io/$PREFIX/ubuntu2204-java17" ubuntu2204 & pids+=($!)

buildkite/docker/push.sh

diff --git a/buildkite/docker/push.sh b/buildkite/docker/push.sh
index 34b1899..323b725 100755
--- a/buildkite/docker/push.sh
+++ b/buildkite/docker/push.sh
@@ -21,7 +21,6 @@
 docker push "gcr.io/$PREFIX/rockylinux8-java11-devtoolset10" &
 docker push "gcr.io/$PREFIX/rockylinux8-releaser" &
 docker push "gcr.io/$PREFIX/debian10-java11" &
-docker push "gcr.io/$PREFIX/debian11-java17" &
 docker push "gcr.io/$PREFIX/debian12" &
 docker push "gcr.io/$PREFIX/debian13" &
 docker push "gcr.io/$PREFIX/ubuntu2204-java17" &

smoke_test/smoke_test.yml

diff --git a/smoke_test/smoke_test.yml b/smoke_test/smoke_test.yml
index a1180d3..b383de7 100644
--- a/smoke_test/smoke_test.yml
+++ b/smoke_test/smoke_test.yml
@@ -2,7 +2,6 @@
 matrix:
   platform:
     # Linux (x86_64) Docker platforms
-    - debian11
     - debian12
     - debian13
     - fedora43