Ubuntu 18.04 in GHA is deprecated from Aug 8,2022

Author: hsbt

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         ruby: [ 3.2, 3.1, '3.0', 2.7, head ]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v3
     - name: Set up Ruby