[CI] Upgrade to macos-15 and xcode 15.4 (#687)

GitHub [has announced](https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/#notice-of-macos-x86_64-intel-architecture-deprecation) that the `macos-13` images are being retired.

Author: straight-shoota

.circleci/config.yml

@@ -60,7 +60,7 @@ jobs:
 
   test-on-osx:
     macos:
-      xcode: 15.0.0
+      xcode: 15.4.0
     steps:
       - with-brew-cache:
           steps:

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-24.04, macos-13, macos-14]
+        os: [ubuntu-24.04, macos-15-intel, macos-15]
         crystal: [latest, nightly]
         include:
           - os: windows-2022