gitlab-ci: update macOS images to Sonoma

The macOS Ventura images we use for GitLab CI runners have been
deprecated. Update them to macOS 14, aka Sonoma.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

.gitlab-ci.yml

@@ -90,10 +90,10 @@ test:osx:
   parallel:
     matrix:
       - jobname: osx-clang
-        image: macos-13-xcode-14
+        image: macos-14-xcode-15
         CC: clang
       - jobname: osx-reftable
-        image: macos-13-xcode-14
+        image: macos-14-xcode-15
         CC: clang
   artifacts:
     paths: