test(CI): switch back to GCC 14

GitHub doesn't yet have GCC 15.

Author: rouson

.github/workflows/CI.yml

@@ -8,7 +8,7 @@ jobs:
 
     env:
       FC: gfortran
-      GCC_V: 15
+      GCC_V: 14
 
     steps:
     - name: Checkout code