Remove Java 21 from CI

Retains 8, 17, 25.

Author: jhy

.github/workflows/build.yml

@@ -10,7 +10,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
         # choosing to run a reduced set of LTS, current, and next, to balance coverage and execution time
-        java: [8, 17, 21, 25]
+        java: [8, 17, 25]
       fail-fast: false
     name: Test JDK ${{ matrix.java }}, ${{ matrix.os }}
     steps: