Use gem instead of runtime dependency

Author: ksss

Gemfile.lock

@@ -3,7 +3,6 @@ PATH
   specs:
     rbs (4.0.0.dev)
       logger
-      rdoc (>= 6.13)
       prism (>= 1.3.0)
 
 PATH

lib/rdoc/discover.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 begin
-  gem 'rdoc', '~> 6.4'
+  gem 'rdoc', '~> 6.13'
   require 'rdoc_plugin/parser'
   module RDoc
     class Parser

rbs.gemspec

@@ -46,6 +46,5 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.required_ruby_version = ">= 3.1"
   spec.add_dependency "logger"
-  spec.add_dependency "rdoc", '>= 6.13'
   spec.add_dependency "prism", ">= 1.3.0"
 end