Update Java version to 21

config.json

@@ -1,4 +1,4 @@
 {
     "judge": "java",
-    "image": "dodona-java17"
+    "image": "dodona-java21"
 }

run

@@ -259,6 +259,6 @@ fi
 dodona close-tab
 
 # Running the tests
-java -Xmx"${memory_limit}k" -cp ".:${worklibs}:${testlibs}:judge.jar:${resources}/properties" -Ddodona.language="${natural_language}" -Ddodona.output_cutoff="${generated_output_cutoff}" dodona.junit.JUnitJSON
+java -Djava.security.manager=allow -Xmx"${memory_limit}k" -cp ".:${worklibs}:${testlibs}:judge.jar:${resources}/properties" -Ddodona.language="${natural_language}" -Ddodona.output_cutoff="${generated_output_cutoff}" dodona.junit.JUnitJSON
 
 dodona close-judgement

test.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env sh
+
+# Test this Judge by running the integration tests in the docker image
+image="dodona-java21:latest"
+
+docker run \
+    --mount type=bind,source=$PWD,destination=/home/runner/workdir,readonly \
+    "$image" \
+    -- ./integration-tests/run