ci: remove unsupported ubuntu 20.04 (#3075)

Author: arrufat

.github/workflows/build_cpp.yml

@@ -121,60 +121,6 @@ jobs:
     - name: Test BLAS bindings
       run: build_blas_bindings/dtest --runall -q
 
-  ubuntu-20_04-gcc-7:
-    runs-on: 'ubuntu-20.04'
-    steps:
-    - uses: actions/checkout@v2
-
-    - name: Install dependencies
-      run: |
-        sudo apt update
-        sudo apt install libwebp-dev make yasm 
-
-    - name: Install gcc 7
-      run: |
-        sudo apt install gcc-7 g++-7
-
-    - name: Cache FFmpeg 3.2.18
-      uses: actions/cache@v3
-      id: cache-ffmpeg3
-      with:
-        path: /home/runner/ffmpeg-n3.2.18_installation
-        key: ffmpeg-n3.2.18_try1
-        
-    - name: Build FFmpeg 3.2.18
-      if: steps.cache-ffmpeg3.outputs.cache-hit != 'true'
-      run: |
-        wget https://github.com/FFmpeg/FFmpeg/archive/refs/tags/n3.2.18.tar.gz
-        tar -xf n3.2.18.tar.gz
-        cd FFmpeg-n3.2.18
-        ./configure --prefix=/home/runner/ffmpeg-n3.2.18_installation --disable-doc --disable-programs
-        make -j4
-        make install
-        cd ..
-
-    - name: Configure
-      run: |
-        export CC=/usr/bin/gcc-7
-        export CXX=/usr/bin/g++-7
-        cmake ${{ github.workspace }}/dlib/test -B build -DCMAKE_PREFIX_PATH=/home/runner/ffmpeg-n3.2.18_installation
-
-    - name: Build just tests
-      run: cmake --build build --config Release --target dtest --parallel 4
-
-    - name: Test
-      run: build/dtest --runall -q
-
-    - name: Build examples, etc
-      run: cmake --build build --config Release --parallel 2
-
-    # Test cmake scrips can build standalone dlib as a shared library
-    - name: Configure dlib as shared library  
-      run: cmake ${{ github.workspace }}/dlib -B build_dlib_shared -DBUILD_SHARED_LIBS=1 -DCMAKE_PREFIX_PATH=/home/runner/ffmpeg-n3.2.18_installation
-
-    - name: Build dlib as shared library
-      run: cmake --build build_dlib_shared --parallel 4
-
   ubuntu-latest-clang-default-avx:
     runs-on: 'ubuntu-latest'
     steps: