Use Ubuntu latest for CI

.github/workflows/test.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
     - name: Set up Ruby