Bump JDK from 17 to 21

Change-Id: I11939b6e3f628531799e69d9815705e7c8234741
diff --git a/.github/workflows/ci_test.yml b/.github/workflows/ci_test.yml
index 4899835..448709c 100644
--- a/.github/workflows/ci_test.yml
+++ b/.github/workflows/ci_test.yml
@@ -29,11 +29,11 @@
           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
 
       - name: Build and install Krill
         run: mvn --file Krill/pom.xml install