Replaced macos-15-intel from macos-13 (#198)

We may test it only `2.5` and `macos-15-intel`. But we have enough
credit to test fiddle gem with Intel macOS under the ruby organization.

Author: hsbt

.github/workflows/ci.yml

@@ -13,9 +13,9 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - macos-13
           - macos-14
           - macos-15
+          - macos-15-intel
           - ubuntu-latest
           - windows-latest
         ruby: