Fix Xcode version

Author: Kohki Miki

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
         include:
         - xcode: "14.3.1"
           macos: macos-13
-        - xcode: "15.3.1"
+        - xcode: "15.4"
           macos: macos-14
         - xcode: "16.0"
           macos: macos-14