Stop testing ruby 2.5 on macOS

setup-ruby no longer supports it.

Author: byroot

.github/workflows/ci.yaml

@@ -18,7 +18,7 @@ jobs:
         ruby: ['2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '3.5', '4.0']
         include:
           - os: macos-latest
-            ruby: '2.5'
+            ruby: '2.6'
           - os: macos-latest
             ruby: '3.2'
           - os: windows-latest