Checkstyle now requires Java 21

Author: AB-xdev

.github/workflows/check-build.yml

@@ -81,7 +81,7 @@ jobs:
     timeout-minutes: 15
     strategy:
       matrix:
-        java: [17]
+        java: [21]
         distribution: [temurin]
     steps:
     - uses: actions/checkout@v6