pom.xml: fix missing Handle jar

It seems Handle jars are not published on Maven Central so we get
this error while packaging:

    [ERROR] Failed to execute goal on project cgspace-java-helpers: Could not resolve dependencies for project io.github.ilri.cgspace:cgspace-java-helpers:jar:7.6-SNAPSHOT: net.handle:handle:jar:9.3.0 was not found in https://repo.maven.apache.org/maven2 during a previous attempt. This failure was cached in the local repository and resolution is not reattempted until the update interval of central has elapsed or updates are forced -> [Help 1]

This is probably related to DSpace 7.x using a vanilla Handle jar
instead of the customized one.

pom.xml

@@ -108,4 +108,17 @@
       </plugins>
     </pluginManagement>
   </build>
+
+  <repositories>
+    <!-- Check Maven Central first (before other repos below) -->
+    <repository>
+        <id>maven-central</id>
+        <url>https://repo.maven.apache.org/maven2</url>
+    </repository>
+    <!-- For Handle Server -->
+    <repository>
+        <id>handle.net</id>
+        <url>https://handle.net/maven</url>
+    </repository>
+  </repositories>
 </project>