Downgrade LLVM from 18 to 16 in CI and update README accordingly.

Author: nkoukpaizan

.github/workflows/spack_default_build.yaml

@@ -129,8 +129,8 @@ jobs:
             packages:
               llvm:
                 externals:
-                - spec: llvm@18
-                  prefix: /usr/lib/llvm-18
+                - spec: llvm@16
+                  prefix: /usr/lib/llvm-16
           EOF
 
       - name: Configure GHCR mirror

README.md

@@ -18,7 +18,7 @@ You should have all of the following installed before installing GridKit™
 	- [Suitesparse](https://github.com/DrTimothyAldenDavis/SuiteSparse) >= 5.x (optional)
 	- [Ipopt](https://github.com/coin-or/Ipopt) >= 3.x (optional)
     - [Enzyme](https://github.com/EnzymeAD/Enzyme) >=0.0.131 (optional)
-        - [LLVM](https://github.com/llvm/llvm-project) >= 18.x 
+        - [LLVM](https://github.com/llvm/llvm-project) 16.x 
 - [CMake](https://cmake.org/) >= 3.12
 - C++ 17 compliant compiler