Add java 21 to build matrix (#301)

Author: Philippus

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        java: [8, 11, 17]
+        java: [8, 11, 17, 21]
         scala: [2.11.x, 2.12.x, 2.13.x, 3.x]
     runs-on: ubuntu-latest
     steps: