run CI with Java 21
diff --git a/.github/workflows/maven.yml b/.github/workflows/maven.yml
index e582f86..d65bd00 100644
--- a/.github/workflows/maven.yml
+++ b/.github/workflows/maven.yml
@@ -25,7 +25,7 @@
     strategy:
       matrix:
         os: [ ubuntu-latest, windows-latest, macOS-latest ]
-        java: [ 11, 17, 20 ]
+        java: [ 11, 17, 21 ]
         dist: [ 'temurin', 'semeru' ]
         exclude:
         - os: macos-latest