Remove Ubuntu 18.04 entirely

.github/workflows/build.yml

@@ -9,9 +9,6 @@ jobs:
       fail-fast: false  # don't cancel other jobs in the matrix if one fails
       matrix:
         include:
-          - runner: ubuntu-20.04
-            container: ubuntu:18.04
-            os_name: ubuntu-18.04
           - runner: ubuntu-20.04
             container: ubuntu:20.04
             os_name: ubuntu-20.04
@@ -39,13 +36,6 @@ jobs:
       image: ${{ matrix.container }}
 
     steps:
-    - name: Node fix for Ubuntu 18.04
-      if: matrix.os_name == 'ubuntu-18.04'
-      run: |
-        echo ':warning: Forcing `node` v16 to be used on Ubuntu 18.04' >> $GITHUB_STEP_SUMMARY
-        echo 'ACTIONS_RUNNER_FORCED_INTERNAL_NODE_VERSION=node16' >> $GITHUB_ENV
-        echo 'ACTIONS_RUNNER_FORCE_ACTIONS_NODE_VERSION=node16' >> $GITHUB_ENV
-
     - uses: actions/checkout@v4
 
     - name: Fetch package lists and install dependencies (Ubuntu)
@@ -79,12 +69,6 @@ jobs:
       if: matrix.os_name == 'ubuntu-23.04'
       run: rm -f /usr/lib/python$(python3 --version | cut -d ' ' -f 2 | cut -d '.' -f 1,2)/EXTERNALLY-MANAGED
 
-    - name: Update Python3 on Ubuntu 18.04
-      if: matrix.os_name == 'ubuntu-18.04'
-      run: |
-        DEBIAN_FRONTEND=noninteractive apt-get -y install python3.8
-        update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 1
-
     - name: Build
       run: make