Stop testing on older rubies

Author: nobu

.github/workflows/test.yml

@@ -8,7 +8,7 @@ jobs:
     with:
       engine: cruby
       versions: '["truffleruby-head"]'
-      min_version: 2.5
+      min_version: 2.7
 
   build:
     needs: ruby-versions
@@ -24,8 +24,6 @@ jobs:
           - ruby: mingw
             os: windows-latest
         exclude:
-          - ruby: 2.5
-            os: macos-latest
           - ruby: head
             os: windows-latest
           - ruby: truffleruby-head