[Build] Update submodules to remote tip only on non-maintenance branches

This prevents maintenance branches from fetching the latest changes on
the master branches of all submodules and therefore failing all builds
early (because the submodule expect a newer parent-pom version).

Follow-up on:
- eclipse-platform#3000

Author: HannesWell

Jenkinsfile

@@ -16,6 +16,14 @@ pipeline {
 		MAVEN_OPTS = '-Xmx4000m'
 	}
 	stages {
+		stage('Update submodules to latest') {
+			when {
+				not { branch 'R*_maintenance' }
+			}
+			steps {
+				sh 'git submodule update --remote'
+			}
+		}
 		stage('Deploy parent-pom and SDK-target') {
 			when {
 				anyOf {