Automate distribution dependency bumps

[abelsromero]

The distribution has several dependencies that depend on manual updates.
To avoid having to keep track manually we could start automating them with dependabot on weekly basis.

I am considering restricting it for now to:

• org.asciidoctor:asciidoctorj-epub3
• org.asciidoctor:asciidoctorj-diagram
• org.asciidoctor:asciidoctorj-diagram-ditaamini
• org.asciidoctor:asciidoctorj-diagram-plantuml
• org.asciidoctor:asciidoctorj-pdf
• org.asciidoctor:asciidoctorj-revealjs
• As well as jruby and jruby-complete for all modules.

If we consider this works for us, we can extend it to other dependencies to reduce manual work.

[abelsromero]

After checking #1228

I think we could narrow branch v2.5.x with (note it excludes asciidoctorj-diagram)

version: 2
updates:
  - package-ecosystem: "gradle"
    directory: "/"
    schedule:
      interval: "weekly"
    labels:
      - "dependencies"
    commit-message:
      prefix: "(v2.5.x)"
    target-branch: "v2.5.x"
    allow:
      - dependency-name: "org.asciidoctor:asciidoctorj-pdf"
      - dependency-name: "org.asciidoctor:asciidoctorj-epub3"
      - dependency-name: "org.asciidoctor:asciidoctorj-diagram-ditaamini"
      - dependency-name: "org.asciidoctor:asciidoctorj-diagram-plantuml"
      - dependency-name: "org.asciidoctor:asciidoctorj-revealjs"

This needs to be on the main branch though...that's how dependabot does it's things