ci: add Java 21 to matrix

Author: sullis

.github/workflows/main.yml

@@ -28,7 +28,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        jdk: [8, 11, 17, 19]
+        jdk: [8, 11, 17, 19, 21]
         os: [ubuntu-latest, windows-latest, macos-latest]
       fail-fast: true
       max-parallel: 4