Use final release URL of Eclipse Platform

Change-Id: Ib6325713d25ad19cdaee760905818bbaf40d6a3d
Signed-off-by: Markus Knauer <mknauer@eclipsesource.com>
diff --git a/releng/org.eclipse.rap.build/pom.xml b/releng/org.eclipse.rap.build/pom.xml
index 39bfe77..aea28a9 100644
--- a/releng/org.eclipse.rap.build/pom.xml
+++ b/releng/org.eclipse.rap.build/pom.xml
@@ -29,7 +29,7 @@
      -->
     <jetty-repo.url>https://download.eclipse.org/webtools/jetty/10.0.6/repository/</jetty-repo.url>
     <orbit-repo.url>https://download.eclipse.org/tools/orbit/downloads/2021-09/</orbit-repo.url>
-    <platform-repo.url>https://download.eclipse.org/eclipse/updates/4.21-I-builds/I20210906-0500/</platform-repo.url>
+    <platform-repo.url>https://download.eclipse.org/eclipse/updates/4.21/R-4.21-202109060500/</platform-repo.url>
     <rap-extra-repo.url>https://download.eclipse.org/rt/rap/base-platforms/3.14/extra-dependencies/</rap-extra-repo.url>
     <license-repo.url>https://download.eclipse.org/cbi/updates/license/</license-repo.url>
     <!-- The build type, N for nighty builds, S for stable builds. S-builds will be signed. -->