Test with macos-15-intel on CI

Author: ankane

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
           - ruby: 3.2
             os: ubuntu-22.04
           - ruby: 3.1
-            os: macos-13
+            os: macos-15-intel
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1