Update JDK to 11 in JDoc workflow

Author: TobiGr

.github/workflows/docs.yml

@@ -15,10 +15,10 @@ jobs:
     steps:
       - uses: actions/checkout@v3
 
-      - name: set up JDK 8
+      - name: set up JDK 11
         uses: actions/setup-java@v3
         with:
-          java-version: '8'
+          java-version: '11'
           distribution: 'temurin'
 
       - name: Cache Gradle dependencies