Use ubuntu-latest runner

Author: marcelotto

.github/workflows/elixir-build-and-test.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   build:
     name: Build and test
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     env:
       MIX_ENV: test
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}