Add llvm21 to ci.yml

Author: yuygfgg

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-24.04, macos-15]
-        llvm: [19, 20]
+        llvm: [19, 20, 21]
     name: Build os=${{ matrix.os }} llvm=${{ matrix.llvm }}
     runs-on: ${{ matrix.os }}
     steps: