GLCI: use an older macOS image (#7405)

Currently, GitLab offers a choice between macos-14-xcode-15 and
macos-15-xcode-16, and Xcode 16.4 is not compatible with the R-bundled
OpenMP runtime. Switch to the older image.

Author: aitap

.gitlab-ci.yml

@@ -337,13 +337,15 @@ test-win-old:
 ## R-release on MacOS
 test-mac-rel:
   <<: *test-mac
+  image: macos-14-xcode-15
   variables:
     R_VERSION: "$R_REL_VERSION"
     R_BIN: "$R_REL_MAC_BIN"
 
 ## R-oldrel on MacOS
 test-mac-old:
   <<: *test-mac
+  image: macos-14-xcode-15
   variables:
     R_VERSION: "$R_OLD_VERSION"
     R_BIN: "$R_OLD_MAC_BIN"