Also exercise JDK16 in CI

Author: vemv

.github/workflows/unit-test.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ windows-latest, ubuntu-latest, macos-latest ]
-        java: [ '8', '11' ]
+        java: [ '8', '11', '16' ]
 
     name: ${{ matrix.os }},jdk ${{ matrix.java }}