GH Actions: Drop Ubuntu 18.04, deprecated

Author: havardAasen

.github/workflows/build.yml

@@ -21,7 +21,7 @@ jobs:
 
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-20.04, ubuntu-latest]
+        os: [ubuntu-20.04, ubuntu-latest]
 
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps: