Build core for Java 17

Android will run core as well, and its runtime stops at Java 17. Pinning
the release makes the compiler reject newer language features and APIs
there, while the desktop modules keep the newest.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ts3-client/core/pom.xml

@@ -14,6 +14,11 @@
     <name>TS3J Client Core</name>
     <description>Frontend-agnostic library: protocol integration, model, audio abstractions</description>
 
+    <properties>
+        <!-- Android runs core too, and it only knows Java 17. -->
+        <maven.compiler.release>17</maven.compiler.release>
+    </properties>
+
     <dependencies>
         <dependency>
             <groupId>com.github.manevolent</groupId>