Include JDK 21 (EA at this point) in CI build

Author: harawata

.github/workflows/ci.yaml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macOS-latest, windows-latest]
-        java: [11, 17, 19, 20]
+        java: [11, 17, 19, 20, 21]
         distribution: ['zulu']
       fail-fast: false
       max-parallel: 4