linux: drop ubuntu 18.04

Ubuntu 18.04 is EOL now, so stop building containers for it.

Also adjust some dates in the documentation

.devcontainer/on-create-command.sh

@@ -41,11 +41,5 @@ apt_get_install \
   openssh-server \
   zsh
 
-# Ubuntu 18.04 doesn't include zsh-autosuggestions
-if ! grep -q "Ubuntu 18.04" /etc/issue &>/dev/null
-then
-  apt_get_install zsh-autosuggestions
-fi
-
 # Start the SSH server so that `gh cs ssh` works.
 sudo service ssh start

.github/workflows/docker.yml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: ["18.04", "20.04", "22.04", "24.04"]
+        version: ["20.04", "22.04", "24.04"]
     steps:
       - name: Checkout
         uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4

docs/Linux-CI.md

@@ -20,12 +20,16 @@ We plan to proceed with regular updates from 2022 onwards. We aim to use the lat
 
 We will start using the latest Ubuntu LTS version for our CI no earlier than 3 months after its release and, ideally, no more than 12 months after its release.
 
-| Distribution | Glibc | GCC | Usage |
+| Distribution | Glibc | GCC | LTS standard security maintenance |
 |---|---|---|---|
 | Ubuntu 14.04 | 2.19 | 4 | From 2014 to 2017 |
 | Ubuntu 16.04 | 2.23 | 5 | From 2017 to 2022 |
-| Ubuntu 22.04 | 2.35 | 11 | From 2022 to 2024 |
-| Ubuntu 24.04 | ? | ? | From 2024 to 2026 |
+| Ubuntu 20.04 | 2.31 | 5 | From 2020 to 2025 |
+| Ubuntu 22.04 | 2.35 | 11 | From 2022 to 2027 |
+| Ubuntu 24.04 | 2.39 | 13 | From 2024 to 2029 |
+| Ubuntu 26.04 | ? | ? | ? |
+
+Source: https://ubuntu.com/about/release-cycle
 
 ## Why always use the latest version?