Update to latest Orbit 4.39 milestone

- Update client5 to 5.6.0 and core5 to 5.4.0.

Author: merks

eclipse.platform.releng.prereqs.sdk/eclipse-sdk-prereqs.target

@@ -29,13 +29,13 @@
       <unit id="org.eclipse.orbit.xml-apis-ext" version="1.0.0.v20240917-0534"/>
 
       <!-- ECF -->
-      <unit id="org.apache.httpcomponents.client5.httpclient5" version="5.5.1.v20250928-1600"/>
+      <unit id="org.apache.httpcomponents.client5.httpclient5" version="5.6.0.v20251221-1000"/>
       <unit id="org.apache.httpcomponents.client5.httpclient5-win" version="5.2.3.v20231203-1619"/>
-      <unit id="org.apache.httpcomponents.core5.httpcore5" version="5.3.6.v20250921-0900"/>
-      <unit id="org.apache.httpcomponents.core5.httpcore5-h2" version="5.3.6.v20250921-0900"/>
+      <unit id="org.apache.httpcomponents.core5.httpcore5" version="5.4.0.v20251215-1000"/>
+      <unit id="org.apache.httpcomponents.core5.httpcore5-h2" version="5.4.0.v20251215-1000"/>
 
       <!-- This is the "normal" Orbit repository is expected to be updated on milestones and releases based on Orbit deliveries. -->
-      <repository location="https://download.eclipse.org/tools/orbit/simrel/orbit-aggregation/release/4.38.0"/>
+      <repository location="https://download.eclipse.org/tools/orbit/simrel/orbit-aggregation/milestone/S202512211802"/>
     </location>
 
     <location includeAllPlatforms="true" includeConfigurePhase="false" includeMode="slicer" includeSource="true" type="InstallableUnit">
@@ -942,7 +942,6 @@
 			  </dependency>
 		  </dependencies>
 	  </location>
-	  <!-- Markdown support for JEP 467 -->
 	  <location includeDependencyDepth="none" includeDependencyScopes="compile" includeSource="true" label="commonmark" missingManifest="error" type="Maven">
 		  <dependencies>
 			  <dependency>