ci: require a java17 agent in Jenkinsfile

The pipeline previously ran on any "light"-labeled agent with no JDK
constraint. Now that the minimum Java version for the core modules is
17, an agent without it installed would fail to compile. Match the
labeling convention already used by Terasology's own Jenkinsfile
(label 'ts-engine && heavy && java17').

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: soloturn

Jenkinsfile

@@ -1,6 +1,6 @@
 pipeline {
     agent {
-        label "light"
+        label "light && java17"
     }
     stages {
         stage('Build') {