[Build] Update, clean-up and use job for Maven publication

Currently the publication of the Maven artifacts of Eclipse happens in
five jobs manually/directly defined in the 'Releng' Jenkins instance:
1. CBIaggregator
2. RepositoryValidator
3. PublishPlatformToMaven, PublishJDTtoMaven and PublishPDEToMaven

Corresponding job-definitions for these jobs already exist in this SCM
(except for the repository-validation), but the jobs generated from them
(in the 'Releng' folder instead of the root) are not used.

This change replaces the existing (unused) multiple job-definitions by a
single Jenkins pipeline that contains all steps and performs the final
publication for all covered projects in a single matrix.
The new pipeline is updated to incorporate all the changes made in the
meantime to the manually maintained publication jobs.

The I-build job is changed to use the new publication job.

With respect to the existing, manually maintained publication job, the
new job is adjusted in the following aspects:

- Rename job to just 'PublishToMaven'. The CBI part by itself is
irrelevant and a tool in the publication process
- Remove documentation about the jobs involved and scripts called from
the job description as it's an implementation detail that can be looked
up in the pipeline if of interest.
- Don't require the 'projects-storage.eclipse.org-bot-ssh' SSH agent for
the aggregation step.
- Only run publication on explicit invocation (from I-builds). Remove
the timer-trigger: Without changes a (re-)publication is not necessary.

JenkinsJobs/Builds/build.jenkinsfile

@@ -373,7 +373,7 @@ spec:
 				}
 			}
 			steps {
-				build job: 'CBIaggregator', parameters: [string(name: 'snapshotOrRelease', value: '-snapshot')], wait: false
+				build job: 'Releng/PublishToMaven', parameters: [choice(name: 'snapshotOrRelease', value: 'snapshot')], wait: false
 			}
 		}
 	}

JenkinsJobs/Releng/FOLDER.groovy

@@ -1,3 +1,62 @@
 folder('Releng') {
   description('Jobs related to routine releng tasks. Some are periodic, some are "manual" jobs ran only when needed.')
 }
+
+pipelineJob('Releng/PublishToMaven'){
+	displayName('Publish to Maven')
+	description('''\
+<p>
+This job uses the <a href="https://github.com/eclipse-cbi/p2repo-aggregator">CBI aggregator</a> to produce a Maven-compatible repository 
+with contents as specified by the <a href="https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/blob/master/eclipse.platform.releng/publish-to-maven-central/SDK4Mvn.aggr">SDK4Mvn.aggr</a> and
+then publishes the artifacts for <code>Eclipse-Platform</code>, <code>JDT</code> and <code>PDE</code> from the output:
+</p>
+<ul>
+<li>
+Snapshots are published to <a href="https://repo.eclipse.org/content/repositories/eclipse-snapshots/">https://repo.eclipse.org/content/repositories/eclipse-snapshots/</a>.
+</li>
+<li>
+Releases are published to <a href="https://repo1.maven.org/maven2/org/eclipse/">Maven central</a> by publishing to a <a href="https://oss.sonatype.org/#stagingRepositories">staging repository</a>.
+</li>
+</ul>
+<p>
+The source repository to be published is specified via the
+<a href="https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/blob/b6c45b1b38b74ad1fa7955e996976da4c259f926/eclipse.platform.releng/publish-to-maven-central/SDK4Mvn.aggr#L5">local</a> and 
+<a href="https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/blob/b6c45b1b38b74ad1fa7955e996976da4c259f926/eclipse.platform.releng/publish-to-maven-central/SDK4Mvn.aggr#L8">remote</a> repository locations.
+<b>
+For a release build, these should specify the release repository location.
+After the release, these should specify the current 4.x-I-Builds.
+</b>
+</p>
+''')
+	parameters { // Define parameters in job configuration to make them available even for the very first build after this job was (re)created.
+		choiceParam('snapshotOrRelease', ['-snapshot' /*default*/, '-release'], '''\
+<p>
+The source repository to be published is specified via the
+<a href="https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/blob/b6c45b1b38b74ad1fa7955e996976da4c259f926/eclipse.platform.releng/publish-to-maven-central/SDK4Mvn.aggr#L5">local</a> and 
+<a href="https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/blob/b6c45b1b38b74ad1fa7955e996976da4c259f926/eclipse.platform.releng/publish-to-maven-central/SDK4Mvn.aggr#L8">remote</a> repository locations 
+the <a href="https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/blob/master/eclipse.platform.releng/publish-to-maven-central/SDK4Mvn.aggr">SDK4Mvn.aggr</a>.
+<b>
+For a release build, these should specify the release repository location.
+After the release, these should specify the current 4.x-I-Builds.
+</b>
+</p>
+<ul>
+<li>
+Snapshots are published to <a href="https://repo.eclipse.org/content/repositories/eclipse-snapshots/">https://repo.eclipse.org/content/repositories/eclipse-snapshots/</a>.
+</li>
+<li>
+Releases are published to <a href="https://repo1.maven.org/maven2/org/eclipse/">Maven central</a> by publishing to a <a href="https://oss.sonatype.org/#stagingRepositories">staging repository</a>.
+</li>
+</ul>
+	''')
+	}
+	definition {
+		cpsScm {
+			lightweight(true)
+			scm {
+				github('eclipse-platform/eclipse.platform.releng.aggregator', 'master')
+			}
+			scriptPath('JenkinsJobs/Releng/publishToMaven.jenkinsfile')
+		}
+	}
+}