Update to Java 21

Change-Id: I68d241c6a1d8da97f212647a3da2ac3793fe4f72
diff --git a/.github/workflows/ci_test.yml b/.github/workflows/ci_test.yml
index cdc4ff4..af8231a 100644
--- a/.github/workflows/ci_test.yml
+++ b/.github/workflows/ci_test.yml
@@ -23,11 +23,11 @@
           repository: KorAP/Koral
           path: Koral
 
-      - name: Set up JDK 17
+      - name: Set up JDK 21
         uses: actions/setup-java@v4
         with:
           distribution: 'temurin'
-          java-version: 17
+          java-version: 21
           cache: 'maven'