Bump RDoc dependency

While we depend on rdoc through the sdoc gem, it only requires a minimum version.

This may be the cause for #47261 broke edgeapi.rubyonrails.org, those docs are generated with Ruby 2.6 or 2.7.

I'd also like to investigate increasing the Ruby version on the doc server, but I feel that has bigger impact.

Gemfile

@@ -49,6 +49,7 @@ end
 
 group :doc do
   gem "sdoc", ">= 2.6.0"
+  gem "rdoc", "~> 6.5"
   gem "redcarpet", "~> 3.2.3", platforms: :ruby
   gem "w3c_validators", "~> 1.3.6"
   gem "rouge"

Gemfile.lock

@@ -567,6 +567,7 @@ DEPENDENCIES
   rack-cache (~> 1.2)
   rails!
   rake (>= 13)
+  rdoc (~> 6.5)
   redcarpet (~> 3.2.3)
   redis (>= 4.0.1)
   redis-namespace