CI: Macos-13 on Intel is deprecated

https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/

Author: larskanis

.github/workflows/binary-gems.yml

@@ -86,7 +86,7 @@ jobs:
           - os: macos-latest
             ruby: "3.4"
             platform: "arm64-darwin"
-          - os: macos-13
+          - os: macos-15-intel
             ruby: "3.4"
             platform: "x86_64-darwin"