CI: Upgrade to macOS 15

Author: acj

.github/workflows/ci.yml

@@ -34,21 +34,21 @@ jobs:
       matrix:
         build:
         - linux-x86_64
-        - macos-13
         - macos-14
+        - macos-15
         - windows
         include:
         - build: linux-x86_64
           os: ubuntu-24.04
           run-tests: 'true'
           target: x86_64-unknown-linux-gnu
-        - build: macos-13
-          os: macos-13
-          run-tests: 'true'
-          target: x86_64-apple-darwin
         - build: macos-14
           os: macos-14
           run-tests: 'true'
+          target: x86_64-apple-darwin
+        - build: macos-15
+          os: macos-15
+          run-tests: 'true'
           target: aarch64-apple-darwin
         - build: windows
           os: windows-2022