Switch Ubuntu 18.04 CI builds to newer Ubuntu versions

Don't use the old Ubuntu version, Ubuntu PPAs are simply too flaky to be
used.

Use the latest available Ubuntu image (26.04) for all builds except
wxDFB as libdirectfb-dev package is not available in this Ubuntu version
any longer, so use 24.04 for this job.

Remove special handling of Ubuntu 18.04 which is not needed any more.

.github/workflows/ci.yml

@@ -78,27 +78,19 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - name: Ubuntu 18.04 wxGTK 2
-            runner: ubuntu-latest
-            container: ubuntu:18.04
+          - name: Ubuntu 26.04 wxGTK 2
+            runner: ubuntu-26.04
             gtk_version: 2
             use_xvfb: true
-          - name: Ubuntu 18.04 wxGTK UTF-8
-            runner: ubuntu-latest
-            container: ubuntu:18.04
+          - name: Ubuntu 26.04 wxGTK UTF-8
+            runner: ubuntu-26.04
             configure_flags: --enable-utf8 --enable-utf8only --enable-monolithic
             use_xvfb: true
-          - name: Ubuntu 18.04 wxGTK 3 static with gcc 4.8
-            runner: ubuntu-latest
-            compiler: g++-4.8
-            container: ubuntu:18.04
-            configure_flags: --disable-shared
-            allow_extra_warnings: true
-            use_xvfb: true
-          - name: Ubuntu 18.04 wxGTK 3 compatible 3.0
-            runner: ubuntu-latest
-            container: ubuntu:18.04
-            configure_flags: --enable-compat30 --enable-pch
+          - name: Ubuntu 20.04 wxGTK gcc 7 static compatible 3.0
+            runner: ubuntu-26.04
+            container: ubuntu:20.04
+            compiler: g++-7
+            configure_flags: --enable-compat30 --enable-pch --disable-shared
             use_xvfb: true
           - name: Ubuntu 20.04 wxGTK 3 with clang
             runner: ubuntu-latest
@@ -127,20 +119,18 @@ jobs:
             use_ubsan: true
             use_xvfb: true
             build_out_of_tree_samples: true
-          - name: Ubuntu 18.04 wxX11
-            runner: ubuntu-latest
-            container: ubuntu:18.04
+          - name: Ubuntu 26.04 wxX11
+            runner: ubuntu-26.04
             configure_flags: --with-x11 --disable-stc
             skip_samples: true
-          - name: Ubuntu 18.04 wxDFB
-            runner: ubuntu-latest
-            container: ubuntu:18.04
+          - name: Ubuntu 24.04 wxDFB
+            # This is the latest Ubuntu version that still has DirectFB in its repositories.
+            runner: ubuntu-24.04
             configure_flags: --with-directfb --disable-stc
             skip_samples: true
             allow_warnings: true
-          - name: Ubuntu 18.04 wxQt
-            runner: ubuntu-latest
-            container: ubuntu:18.04
+          - name: Ubuntu 26.04 wxQt
+            runner: ubuntu-26.04
             configure_flags: --with-qt --without-opengl
             skip_samples: true
             use_xvfb: true
@@ -159,16 +149,6 @@ jobs:
             ubuntu:*)
               export DEBIAN_FRONTEND=noninteractive
 
-              if [ '${{ matrix.container }}' = 'ubuntu:18.04' ]; then
-                # First get the package containing /usr/bin/apt-add-repository.
-                apt-get update -qq
-                apt-get install -qq software-properties-common
-
-                # Git 2.17 in the official repository is too old to checkout
-                # submodules using it, so get a newer version.
-                apt-add-repository ppa:git-core/ppa
-              fi
-
               compiler=${{ matrix.compiler }}
               case "$compiler" in
                 clang++*)
@@ -194,22 +174,6 @@ jobs:
               #   - xvfb is used for running the GUI tests.
               apt-get update -qq
               apt-get install -qq coreutils ${compiler_packages} git make pkg-config sudo xvfb
-
-              # Checkout action doesn't work under Ubuntu 18.04 any more, so
-              # run checkout manually.
-              git config --global --add safe.directory $GITHUB_WORKSPACE
-              git config --global gc.auto 0
-              git init $GITHUB_WORKSPACE
-              cd $GITHUB_WORKSPACE
-              git remote add origin $GITHUB_SERVER_URL/$GITHUB_REPOSITORY
-              git fetch --depth=1 origin $GITHUB_REF
-              git checkout FETCH_HEAD
-              git submodule update --init --depth=1 --recursive
-              git log -1 --format='%H'
-
-              # We can't easily install Go either, so skip the tests requiring
-              # httpbin.
-              echo WX_TEST_WEBREQUEST_URL=0 >> $GITHUB_ENV
               ;;
 
             '')
@@ -221,13 +185,11 @@ jobs:
           esac
 
       - name: Checkout
-        if: matrix.container != 'ubuntu:18.04'
         uses: actions/checkout@v6
         with:
           submodules: 'recursive'
 
       - name: Install CCache
-        if: matrix.container != 'ubuntu:18.04'
         uses: hendrikmuhs/ccache-action@v1.2.23
         with:
           key: ${{ matrix.name }}
@@ -370,7 +332,6 @@ jobs:
         if: ${{ github.event_name == 'workflow_dispatch' && inputs.debug_tests }}
 
       - name: Setup Go
-        if: matrix.container != 'ubuntu:18.04'
         uses: actions/setup-go@v6
         with:
           go-version: '1'