Use ubuntu-latest for actions build runs

20.04 is no longer supported.

Author: Menno Finlay-Smits

.github/workflows/main.yml

@@ -9,7 +9,7 @@ on:
   workflow_dispatch:
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     timeout-minutes: 10
     strategy:
       fail-fast: false