Build weekly releases with Java 11

[basil]

Preparation for doing weekly releases with Java 11. This PR cannot be merged until after Jenkins core adopts Java 11 (currently scheduled for June for weekly releases). Implements Proposal A from jenkins-infra/helpdesk#2945 (comment). Additionally adds printing of the Java version being used to ease debugging.

Testing done

I did not run this actual Pipeline job, but I tested my syntax with the following simple job:

pipeline {
  agent any
  parameters {
    choice(
        choices: ['weekly', 'stable', 'stable-rc', 'security'],
        description: 'Define which Jenkins Release we are packaging for. https://github.com/jenkins-infra/release/tree/master/profile.d',
        name: 'RELEASE_PROFILE'
        )
  }

  environment {
    JAVA_VERSION = "${params.RELEASE_PROFILE == 'weekly' ? '11' : '8'}"
  }

  stages {
    stage('Build') {
      environment {
        JAVA_HOME = "/opt/jdk-${env.JAVA_VERSION}"
        PATH = "${env.JAVA_HOME}/bin:${env.PATH}"
      }
      steps {
        sh 'env | grep JAVA'
        sh 'env | grep PATH'
      }
    }
  }
}

I verified that this simple job printed the expected values for JAVA_HOME and PATH when run with both weekly and stable as the input parameters.

[basil]

Apparently in jenkins-infra/docker-packaging#36 we decided to start building all releases with Java 11, even releases that only require Java 8.

[dduportal]

Sorry @basil for missing this one :(