Merge branch 'jc/ci-use-macos-14'

The version of macos image used in GitHub CI has been updated to
macos-14, as the macos-13 that we have been using got deprecated.

* jc/ci-use-macos-14:
  GitHub CI: macos-13 images are no more

.github/workflows/main.yml

@@ -313,16 +313,16 @@ jobs:
         vector:
           - jobname: osx-clang
             cc: clang
-            pool: macos-13
+            pool: macos-14
           - jobname: osx-reftable
             cc: clang
-            pool: macos-13
+            pool: macos-14
           - jobname: osx-gcc
             cc: gcc-13
-            pool: macos-13
+            pool: macos-14
           - jobname: osx-meson
             cc: clang
-            pool: macos-13
+            pool: macos-14
     env:
       CC: ${{matrix.vector.cc}}
       CC_PACKAGE: ${{matrix.vector.cc_package}}