Use release repository url for GEF Classic 3.19

The milestone repository will be deleted with the upcoming release. Both
repositories are identical, so this change merely avoids potential build
errors during the 2024-06 release cycle

gef.aggrcon

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="ASCII"?>
 <aggregator:Contribution xmi:version="2.0" xmlns:xmi="http://www.omg.org/XMI" xmlns:aggregator="http://www.eclipse.org/cbi/p2repo/2011/aggregator/1.1.0" label="GEF">
-  <repositories location="https://download.eclipse.org/tools/gef/classic/milestone/S202402212051" description="GEF-Classic releases">
+  <repositories location="https://download.eclipse.org/tools/gef/classic/release/3.19.0" description="GEF-Classic releases">
     <features name="org.eclipse.gef.sdk.feature.group">
       <categories href="simrel.aggr#//@customCategories[identifier='Modeling']"/>
     </features>