Build on ubuntu-20.04 image (18.04 is obsolete)

Author: dktapps

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
 
     strategy: