Test ubuntu 20.04

Author: jvdp1

.github/workflows/CI.yml

@@ -19,7 +19,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-13]
+        os: [ubuntu-20.04, macos-13]
         toolchain:
           - {compiler: gcc, version: 10}
           - {compiler: gcc, version: 11}