CI Tests: Upgrade to Java 11

Upgrade CI testing environment from Java 8 to Java 11.
diff --git a/.github/workflows/ci_test.yml b/.github/workflows/ci_test.yml
index 635eccd..42ec057 100644
--- a/.github/workflows/ci_test.yml
+++ b/.github/workflows/ci_test.yml
@@ -23,10 +23,10 @@
           repository: KorAP/Koral
           path: Koral
 
-      - name: Set up JDK 1.8
+      - name: Set up JDK 11
         uses: actions/setup-java@v1
         with:
-          java-version: 1.8
+          java-version: 11
 
       - name: Build and install Krill
         run: mvn --file Krill/pom.xml install