Switch macos-13 CI jobs to macos-15-intel

macOS 13 runner images will be retired by 2025-12-04:
https://github.blog/changelog/2025-09-19-github-actions-macos-13-runner-image-is-closing-down/.

Author: tautschnig

.github/workflows/pull-request-check-rust-api.yaml

@@ -69,8 +69,8 @@ jobs:
           CBMC_INCLUDE_DIR=../../${{env.default_include_dir}} CBMC_LIB_DIR=../../${{env.default_build_dir}}/lib CBMC_VERSION=$VERSION cargo test -- --test-threads=1
 
 
-  check-macos-13-cmake-clang-rust:
-    runs-on: macos-13
+  check-macos-15-intel-cmake-clang-rust:
+    runs-on: macos-15-intel
     steps:
       - uses: actions/checkout@v5
         with:

.github/workflows/pull-request-checks.yaml

@@ -642,8 +642,8 @@ jobs:
         run: cd build; ctest . -V -L THOROUGH -j${{env.linux-vcpus}}
 
   # This job takes approximately 39 to 69 minutes
-  check-macos-13-make-clang:
-    runs-on: macos-13
+  check-macos-15-intel-make-clang:
+    runs-on: macos-15-intel
     steps:
       - uses: actions/checkout@v5
         with:

.github/workflows/release-packages.yaml

@@ -220,7 +220,7 @@ jobs:
           SLACK_MESSAGE: "${{ job.status == 'success' && 'Ubuntu 22.04 package built and uploaded successfully' || 'Ubuntu 22.04 package build failed' }}"
 
   homebrew-pr:
-    runs-on: macos-13
+    runs-on: macos-15-intel
     steps:
       - name: Get release tag name
         # The GITHUB_REF we get has refs/tags/ in front of the tag name so we