fix failing macos test

Looks like actions/runner-images#10703 took
away support for xcode 14. That is why the latest action failed. There
an active PR adding support for xcode 16 to the macos runner
(actions/runner-images#10967). This commit will
fail until that has been merged, but it is probably worth waiting for.

.github/workflows/compilers.yml

@@ -54,7 +54,7 @@ jobs:
   xcode:
     strategy:
       matrix:
-        xcode_version: [ '14.3.1', '15.4' ]
+        xcode_version: [ '15.4' ]
         build_type: [ Debug, Release ]
         cxx_version: [ 14, 17, 20, 23 ]
     runs-on: macos-latest