Update to ubuntu-latest for building images

Author: tekante

.github/workflows/default.yml

@@ -18,7 +18,7 @@ env:
 jobs:
   build:
     name: "Build: ${{ matrix.version }}"
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     strategy:
       fail-fast: false # Don't cancel other jobs if one fails