Bump LLVM version to 21 in CI

Author: victoryforce

.github/workflows/ci.yml

@@ -61,7 +61,7 @@ jobs:
           - "ubuntu:25.10"
         compiler:
           - { compiler: GNU15,  CC: gcc-15,   CXX: g++-15,     packages: gcc-15 g++-15 }
-          - { compiler: LLVM20, CC: clang-20, CXX: clang++-20, packages: clang-20 libomp-20-dev llvm-20-dev libc++-20-dev libc++abi1-20 lld-20 clang-tools-20 mlir-20-tools libmlir-20-dev}
+          - { compiler: LLVM21, CC: clang-21, CXX: clang++-21, packages: clang-21 libomp-21-dev llvm-21-dev libc++-21-dev libc++abi1-21 lld-21 clang-tools-21 mlir-21-tools libmlir-21-dev}
         btype:
           - Release
         target: