CI: add a target with older Ubuntu to test older compilers

Author: robUx4

.github/workflows/ubuntu-20.04.yaml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        cxx: ['g++-9', 'llvm-12']
+        cxx: ['g++', 'clang-10']
     env:
       CMAKE_OPTIONS: -DDEV_MODE=ON -DBUILD_TESTING=ON
       CMAKE_MATROSKA_OPTIONS: -DBUILD_EXAMPLES=ON
@@ -46,91 +46,3 @@ jobs:
 
       - name: Test installation
         run: cmake --install _build --prefix ${GITHUB_WORKSPACE}/_built
-
-  Linux-Clang:
-    runs-on: ubuntu-20.04
-    strategy:
-      matrix:
-        cxx: ['7', '18']
-    env:
-      CMAKE_OPTIONS: -DDEV_MODE=ON -DBUILD_TESTING=ON -DCMAKE_CXX_FLAGS="-stdlib=libc++"
-      CMAKE_MATROSKA_OPTIONS: -DBUILD_EXAMPLES=ON
-    steps:
-      - name: Install libc++
-        run: |
-          sudo apt install -y libc++abi-${{matrix.cxx}}-dev libc++-${{matrix.cxx}}-dev
-
-      - name: Get pushed code
-        uses: actions/checkout@v4
-
-      - name: Checkout libebml
-        uses: actions/checkout@v4
-        with:
-          repository: Matroska-Org/libebml
-          path: libebml
-          # minimum version we support ref: 'release-1.4.3'
-
-      - name: Configure libebml
-        run: cmake -S libebml -B libebml/_build ${{ env.CMAKE_OPTIONS }}
-
-      - name: Build libebml
-        run: cmake --build libebml/_build --parallel
-
-      - name: Install libebml
-        run: cmake --install libebml/_build --prefix ${GITHUB_WORKSPACE}/_built
-
-      - name: Configure
-        run: cmake -S . -B _build  ${{ env.CMAKE_OPTIONS }} ${{ env.CMAKE_MATROSKA_OPTIONS }} -DEBML_DIR="${GITHUB_WORKSPACE}/_built/lib/cmake/EBML"
-
-      - name: Build
-        run: cmake --build _build --parallel
-
-      - name: Test installation
-        run: cmake --install _build --prefix ${GITHUB_WORKSPACE}/_built
-
-  Alpine:
-    runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        platform: ['armhf']
-    env:
-      CMAKE_OPTIONS: -DDEV_MODE=ON -DBUILD_TESTING=ON
-      CMAKE_MATROSKA_OPTIONS: -DBUILD_EXAMPLES=ON
-    defaults:
-      run:
-        shell: alpine.sh {0}
-    steps:
-      - name: Get pushed code
-        uses: actions/checkout@v4
-
-      - uses: jirutka/setup-alpine@v1
-        with:
-          branch: edge
-          arch: ${{matrix.platform}}
-          packages: >
-            build-base cmake
-
-      - name: Checkout libebml
-        uses: actions/checkout@v4
-        with:
-          repository: Matroska-Org/libebml
-          path: libebml
-          # minimum version we support ref: 'release-1.4.3'
-
-      - name: Configure libebml
-        run: cmake -S libebml -B libebml/_build ${{ env.CMAKE_OPTIONS }}
-
-      - name: Build libebml
-        run: cmake --build libebml/_build --parallel
-
-      - name: Install libebml
-        run: cmake --install libebml/_build --prefix ${GITHUB_WORKSPACE}/_built
-
-      - name: Configure
-        run: cmake -S . -B _build  ${{ env.CMAKE_OPTIONS }} ${{ env.CMAKE_MATROSKA_OPTIONS }} -DEBML_DIR="${GITHUB_WORKSPACE}/_built/lib/cmake/EBML"
-
-      - name: Build
-        run: cmake --build _build --parallel
-
-      - name: Test installation
-        run: cmake --install _build --prefix ${GITHUB_WORKSPACE}/_built