Update asciidoc dependencies for Ruby 3

What does this MR do?

To enable GitLab to run on Ruby 3, we need to update a number of dependencies. Most of those are due to deprecations that would now break, such as keyword arguments. In this case, it should resolve https://gitlab.com/gitlab-org/gitlab/-/jobs/1401110409/artifacts/file/deprecations/lib/gitlab/asciidoc.yml

This MR updates asciidoctor and related dependencies:

• asciidoctor 2.0.15 (was: 2.0.12)
  • https://github.com/asciidoctor/asciidoctor/blob/master/CHANGELOG.adoc#2015-2021-04-27---mojavelinux
• asciidoctor-kroki 0.5.0 (was: 0.3.0)
  • https://github.com/Mogztter/asciidoctor-kroki/releases/tag/ruby-v0.5.0

I also dropped a monkey patch that according to the code comment is not necessary anymore, since the problem was fixed upstream.

Screenshots or Screencasts (strongly suggested)

Before:

$ bin/spring rspec spec/lib/gitlab/asciidoc_spec.rb
...
1) Gitlab::Asciidoc without project with Kroki enabled converts a graphviz diagram to image
     Failure/Error: raise RaiseDisallowedDeprecation.new(test, current_deprecations)
     
     DeprecationToolkitEnv::DeprecationBehaviors::SelectiveRaise::RaiseDisallowedDeprecation:
       Disallowed deprecations detected while running test #<RSpec::Core::Example "converts a graphviz diagram to image">:
     
       DEPRECATION WARNING: /data/cache/bundle-2.7.2/ruby/2.7.0/gems/asciidoctor-2.0.15/lib/asciidoctor/extensions.rb:1398: warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call
       /data/cache/bundle-2.7.2/ruby/2.7.0/gems/asciidoctor-kroki-0.4.0/lib/asciidoctor/extensions/asciidoctor_kroki/extension.rb:22: warning: The called method `initialize' is defined here
        (called from block (2 levels) in render at /home/git/gitlab/lib/gitlab/asciidoc.rb:60)

After:

bin/spring rspec spec/lib/gitlab/asciidoc_spec.rb
...
Finished in 23.91 seconds (files took 1.52 seconds to load)
65 examples, 0 failures

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #328031 (closed)