Update clang version matrix in self-test workflow

Author: shenxianpeng

.github/workflows/self-test.yml

@@ -31,7 +31,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, macos-latest, windows-latest ]
-        clang-version: ['8','9','10', '11', '12', '13', '14', '15', '16', '17', '18', '19', '20']
+        clang-version: ['9', '10', '11', '12', '13', '14', '15', '16', '17', '18', '19', '20', '21']
       fail-fast: false
     runs-on: ${{ matrix.os }}
     steps: