feat(jenkins): remove deprecated 'jenkins' package

[CryptoRodeo]

Hey, I just made a Pull Request!

Remove the deprecated jenkins NPM package and replace it with a built-in lightweight client.

The goal of this client is for it to be "drop-in" and minimize changes where the original client was called.

Should fix #4984

Basic Jenkins plugin operations

These still work as expected (at least for me 😄)

jenkins-backstage-demo-basic.mp4

Scaffolder functionality

The Scaffolder still works the same:

jenkins-scaffolder-demo-basic.mp4

For reference here is the template I used:

apiVersion: scaffolder.backstage.io/v1beta3
kind: Template
metadata:
  name: debug-jenkins
  title: Debug Jenkins
spec:
  owner: guests
  type: service
  system: examples
  lifecycle: experimental
  steps:
    - id: create
      name: Create Jenkins job
      action: jenkins:job:create
      input:
        jobName: demo
        jobXml: |
          <flow-definition plugin="workflow-job">
            <definition class="org.jenkinsci.plugins.workflow.cps.CpsFlowDefinition" plugin="workflow-cps">
              <script>pipeline { agent any; stages { stage('hi'){steps{ echo 'hello' } } } }</script>
              <sandbox>true</sandbox>
            </definition>
          </flow-definition>

    - id: clone
      name: Clone Jenkins Job
      action: jenkins:job:copy
      input:
        sourceJobName: demo
        targetJobName: demoClone

    - id: build
      name: Trigger build
      action: jenkins:job:build
      input:
        jobName: demo

    - id: disable
      name: Disable job
      action: jenkins:job:disable
      input:
        jobName: demo

    - id: enable
      name: Re-enable job
      action: jenkins:job:enable
      input:
        jobName: demo

    - id: destroy
      name: Destroy job
      action: jenkins:job:destroy
      input:
        jobName: demo

    - id: destroy copy
      name: Destroy job copy
      action: jenkins:job:destroy
      input:
        jobName: demoClone

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-jenkins-backend	workspaces/jenkins/plugins/jenkins-backend	minor	v0.20.0
@backstage-community/plugin-jenkins-common	workspaces/jenkins/plugins/jenkins-common	minor	v0.12.0
@backstage-community/plugin-scaffolder-backend-module-jenkins	workspaces/jenkins/plugins/scaffolder-backend-module-jenkins	minor	v0.14.0

[CryptoRodeo]

So far no issues connecting to my Jenkins instance:

basic-jenkins-test.mp4

Jenkins scaffolder also works:

jenkins-scaffolder-test.mp4