CI: build on JDK 25; update Gradle to 9.4.1

Gradle 9 is required for JDK 25 and now needs the JUnit Platform launcher
declared explicitly. The CI matrix moves to 17/21/25 (Gradle 9 requires JDK 17+
to run; the library still targets Java 11).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: FlorianKirmaier

.github/workflows/main.yml

@@ -6,7 +6,7 @@ jobs:
   build:
     strategy:
       matrix:
-        jdk: [11, 17, 21]
+        jdk: [17, 21, 25]
 
     runs-on: ubuntu-latest