CI: disable Ubuntu 20.04 builds

Ubuntu 20.04 has been disabled on GitHub. Stop using it for building
images.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

Author: lumag

.github/workflows/build.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ ubuntu-24.04, ubuntu-22.04, ubuntu-20.04, macos-latest ]
+        os: [ ubuntu-24.04, ubuntu-22.04, macos-latest ]
     runs-on: ${{ matrix.os }}
 
     steps: