CI: Remove flang on macos-13

Homebrew no longer provides a binary bottle for llvm or flang on macos-13

Author: bonachea

.github/workflows/build.yml

@@ -29,9 +29,6 @@ jobs:
 
           # --- LLVM flang coverage ---
 
-          - os: macos-13
-            compiler: flang
-            version: 21
           - os: macos-14
             compiler: flang
             version: 21