(https://github.com/ruby/irb/pull/897)

Some features of irb do not work properly when using the old rdoc.
I have compared several major versions and found that it works as intended from 4.0.0.

This problem occurs when there is a Gemfile.lock is installed with the old rdoc.
I don't know why this Gemfile.lock installs an older rdoc than the ruby bundled rdoc,
but specifying the version in the gemspec will at least prevent the problem.

NOTE: ruby/irb#704 problem does not occur with this change.

The following is test code.

```
 ### Usage: ruby __FILE__.rb
 # # input RDoc and Tab
 # >> RDoc<Tab>
 #
 ### Expect: Display document of RDoc
 ### Actual: <internal:marshal>:34:in `load': instance of RDoc::Constant needs to have method `marshal_load' (TypeError)
require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }
  gem 'irb'
  # gem 'rdoc', '~> 4.0.0'
  gem 'rdoc', '~> 3.12.0'
end

require 'rdoc'
require 'irb'

IRB.start
```

https://github.com/ruby/irb/commit/1a1fbba020

(https://github.com/ruby/irb/pull/834)

https://github.com/ruby/irb/commit/eff8d0d46a

(https://github.com/ruby/irb/pull/704)

1. The newer versions of rdoc requires pysch 4.0+, which could break apps
   using Ruby 3.0 or 2.7. #703 has more detailed explanation on this.
2. We actually don't use any version-specific rdoc APIs. So having a version
    requirement is not necessary atm.

https://github.com/ruby/irb/commit/3e6ba78c42

(https://github.com/ruby/irb/pull/702)

Reline 0.3.8 reduces the chance of having a deadlock with the debugger
while using the new `irb:rdbg` integration.

https://github.com/ruby/irb/commit/9b8c56b7d4

(https://github.com/ruby/irb/pull/648)

IRB already has several features that rely on rdoc, such as:

- Autocompletion's document dialog
- Autocompletion's `PerfectMatchedProc`
- The `show_doc` command
- Easter egg

And we could use its pager more in the future too. So it makes sense to
declare rdoc as a dependency instead of relying on the one bundled with
Ruby.

https://github.com/ruby/irb/commit/4dffbb1dd3

(https://github.com/ruby/irb/pull/632)

* Require Reline 0.3.6 or higher

"Multi-line multibyte regular expression raises exception" (https://github.com/ruby/irb/issues/627) is fixed in Reline 0.3.6.

* Remove unnecessary conditional

(https://github.com/ruby/irb/pull/555)

* Remove all Ruby 2.6 support

* Drop Ruby 2.6 specific testing conditions

* Only run Ruby 2.7+ on CI

* Bump Ruby requirement to 2.7+

https://github.com/ruby/irb/commit/3f714b616c

rubygems.org
(https://github.com/ruby/irb/pull/532)

https://github.com/ruby/irb/commit/f01c2a6ac1

Because it has reached EOL for more than 1.5 years and it won't be
supported by the next reline version either.

https://github.com/ruby/irb/commit/a6bfa7b2e6