Drop Ubuntu 18.04 support

Author: wataru86

.github/workflows/test.yml

@@ -36,7 +36,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ['ubuntu-18.04', 'ubuntu-latest', 'macos-latest']
+        os: ['ubuntu-latest', 'macos-latest']
         ruby: ['3.0', '3.1']
         experimental: [false]
         include: