Update CI to install llvm 21

Author: AmrDeveloper

.github/workflows/ci.yaml

@@ -26,7 +26,7 @@ jobs:
     steps:
       - run: wget https://apt.llvm.org/llvm.sh
       - run: chmod +x llvm.sh
-      - run: sudo ./llvm.sh 20
+      - run: sudo ./llvm.sh 21
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@stable
       - run: cargo clippy -- -D warnings