Update CI build matrix (drop JDK 19 and 21-ea, add 21)

Author: matthiasblaesing

.github/workflows/ci.yaml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        java: [8, 11, 17, 19, 20-ea]
+        java: [8, 11, 17, 21]
         os: [ubuntu-latest, macos-latest]
       # Run all tests even if one fails
       fail-fast: false