Also drop EOL 20.04

Author: benemer

.github/workflows/cpp.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
+        os: [ubuntu-22.04, ubuntu-24.04]
     steps:
       - uses: actions/checkout@v3
       - name: Setup cmake
@@ -35,7 +35,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04, ubuntu-22.04, ubuntu-24.04]
+        os: [ubuntu-22.04, ubuntu-24.04]
 
     steps:
       - uses: actions/checkout@v3