ci: build platform gems for Ruby 4.0

Author: kateinoigakukun

.github/workflows/build-gems.yml

@@ -27,7 +27,7 @@ jobs:
           supported-ruby-platforms: |
             exclude: [arm-linux, x64-mingw32, aarch64-mingw-ucrt]
           stable-ruby-versions: |
-            exclude: [head, "4.0"]
+            exclude: [head]
 
   cross-gem:
     name: Compile native gem for ${{ matrix.ruby-platform }}