ci : change macos-13 to macos-15-intel (ggml-org#16401)

This commit updates the macos-13 runners to macos-15-intel.

The motivation for this changes is the macos-13 runners are scheduled
to be retired on 2025-12-04.

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

Author: danbev

.github/workflows/build.yml

@@ -97,7 +97,7 @@ jobs:
           ctest -L 'main|curl' --verbose --timeout 900
 
   macOS-latest-cmake-x64:
-    runs-on: macos-13
+    runs-on: macos-15-intel
 
     steps:
       - name: Clone

.github/workflows/release.yml

@@ -75,7 +75,7 @@ jobs:
           name: llama-bin-macos-arm64.zip
 
   macOS-x64:
-    runs-on: macos-13
+    runs-on: macos-15-intel
 
     steps:
       - name: Clone