gitlab-ci: update to macOS 15 images

The macos-14-xcode-15 images for GitLab's macOS runners have been
deprecated. Update to macOS 15, which is our current stable version.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

.gitlab-ci.yml

@@ -101,13 +101,13 @@ test:osx:
   parallel:
     matrix:
       - jobname: osx-clang
-        image: macos-14-xcode-15
+        image: macos-15-xcode-16
         CC: clang
       - jobname: osx-reftable
-        image: macos-14-xcode-15
+        image: macos-15-xcode-16
         CC: clang
       - jobname: osx-meson
-        image: macos-14-xcode-15
+        image: macos-15-xcode-16
         CC: clang
   artifacts:
     paths: