ci: Remove deprecated Ubuntu 20.04 from runners

The Ubuntu 20.04-based runners have been deprecated by GitHub at the
beginning of this month, and are expected to go fully unsupported by
2025-04-01. Let's remove it from the workflow file.

Link: actions/runner-images#11101
Signed-off-by: Quentin Monnet <qmo@kernel.org>

Author: qmonnet

.github/workflows/build.yaml

@@ -21,7 +21,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
+        os: [ubuntu-22.04, ubuntu-24.04]
     runs-on: ${{ matrix.os }}
     env:
       FEATURES: .llvm and .skeletons
@@ -32,12 +32,6 @@ jobs:
         with:
           submodules: true
 
-      - name: Use clang-12 and not clang-11 for older Ubuntu
-        if: matrix.os == 'ubuntu-20.04'
-        run: |
-          sudo apt-get remove -y clang-11 llvm-11
-          sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-12 50
-
       - name: Install dependencies
         run: |
           sudo apt-get update