Eventually re-add jruby to the CI matrix

[postmodern]

Ronin now requires ruby >= 3.0. Re-enable JRuby in the CI matrix once it achieves 3.0 support.

[postmodern]

Blocked by some weird nokogiri parsing differences under JRuby:

  1) Ronin::Web::HTML.parse must parse an HTML String and return a Nokogiri::HTML::Document
     Failure/Error: expect(doc.at('body').inner_text).to eq("Hello")

       expected: "Hello"
            got: "Hello\n"

       (compared using ==)

       Diff:
         <The diff is empty, are your objects producing identical `#inspect` output?>
     # ./spec/html_spec.rb:18:in `block in <main>'

  2) Ronin::Web.html should be able to parse HTML
     Failure/Error: expect(doc.at('body').inner_text).to eq("Hello")

       expected: "Hello"
            got: "Hello\n      "

       (compared using ==)

       Diff:
       @@ -1,2 +1,3 @@
        Hello
       +      

     # ./spec/web_spec.rb:19:in `block in <main>'