[RelEng] Streamline Y-builds p2 repositories

Populate a new Y-builds composite repo upon creation during the
preparation with a redirect to the previous release's composite
p2-repository. This smoothens the transition to a new dev-stream.

And remove the generic latest Y-build composite:
https://download.eclipse.org/eclipse/updates/Y-builds/
The slightly different schedule of I- and Y-builds at the end of a
stream repeatedly lead to confusion and using specific repositories
clarifies that.

Author: HannesWell

JenkinsJobs/Releng/prepareNextDevCycle.jenkinsfile

@@ -299,7 +299,8 @@ pipeline {
 				]
 				build job: 'Releng/modifyP2CompositeRepository', wait: true, propagate: true, parameters: [
 					string(name: 'repositoryPath', value: "eclipse/updates/${NEXT_RELEASE_VERSION}-Y-builds"),
-					string(name: 'repositoryName', value: "Eclipse ${NEXT_RELEASE_VERSION} Beta Java builds")
+					string(name: 'repositoryName', value: "Eclipse ${NEXT_RELEASE_VERSION} Beta Java builds"),
+					string(name: 'add', value: "https://download.eclipse.org/eclipse/updates/${PREVIOUS_RELEASE_VERSION}-Y-builds/"),
 				]
 				build job: 'Releng/modifyP2CompositeRepository', wait: true, propagate: true, parameters: [
 					string(name: 'repositoryPath', value: "eclipse/updates/${NEXT_RELEASE_VERSION}"),
@@ -313,12 +314,6 @@ pipeline {
 					string(name: 'sizeLimit', value: '1'), // Clear all previous children
 					string(name: 'add', value: "https://download.eclipse.org/eclipse/updates/${NEXT_RELEASE_VERSION}-I-builds/"),
 				]
-				build job: 'Releng/modifyP2CompositeRepository', wait: true, propagate: true, parameters: [
-					string(name: 'repositoryPath', value: "eclipse/updates/Y-builds"),
-					string(name: 'repositoryName', value: "Eclipse latest Beta Java builds"),
-					string(name: 'sizeLimit', value: '1'), // Clear all previous children
-					string(name: 'add', value: "https://download.eclipse.org/eclipse/updates/${NEXT_RELEASE_VERSION}-Y-builds/"),
-				]
 			}
 		}
 		stage('Deploy parent-pom and SDK-target') {