ci: use macOS-15

Author: carlosmn

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
       fail-fast: false
       matrix:
         ruby: [ '2.6', '2.7', '3.0', '3.1', '3.2' ]
-        os: [ macOS-13 ]
+        os: [ macOS-15 ]
     runs-on: ${{ matrix.os }}
 
     name: Ruby ${{ matrix.ruby }} on ${{ matrix.os }}