Remove Ubuntu 20.04 CI target

[why]
The Github runners are no longer available.

Signed-off-by: Lars Froehlich <lars.froehlich@desy.de>

Author: alt-graph

.github/workflows/build_and_run_tests.yml

@@ -28,7 +28,7 @@ jobs:
     name: Build and Tests
     strategy:
       matrix:
-        os: [ ubuntu-20.04, ubuntu-22.04, ubuntu-24.04 ]
+        os: [ ubuntu-22.04, ubuntu-24.04 ]
     runs-on: ${{ matrix.os }}
     steps:
     - run: |