Allow rdoc from ruby 2.5 to match requirement

--- a/Gemfile
+++ b/Gemfile
@@ -156,7 +156,7 @@
 gem 'commonmarker', '~> 0.21'
 gem 'kramdown', '~> 2.3','>= 2.3.1'
 gem 'RedCloth', '~> 4.3', '>= 4.3.2'
-gem 'gitlab-rdoc', '~> 6.3', '>= 6.3.2', require: 'rdoc' # We need this fork until rdoc releases a new version. See https://gitlab.com/gitlab-org/gitlab/-/issues/334695
+gem 'rdoc', '~> 6.0', require: 'rdoc' # See https://gitlab.com/gitlab-org/gitlab/-/issues/334695
 gem 'org-ruby', '~> 0.9.12'
 gem 'creole', '~> 0.5.0'
 gem 'wikicloth', '0.8.1'