Update rdoc requirement from ~> 6.0, < 6.4.0 to ~> 6.16

Updates the requirements on [rdoc](https://github.com/ruby/rdoc) to permit the latest version.
- [Release notes](https://github.com/ruby/rdoc/releases)
- [Changelog](https://github.com/ruby/rdoc/blob/master/History.rdoc)
- [Commits](ruby/rdoc@v6.0.0...v6.16.0)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-version: 6.16.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -33,7 +33,7 @@ group(:features) do
   #gem 'ruby-shadow', '~> 2.5', require: false, platforms: [:ruby]
   gem 'minitar', '~> 1.0', require: false
   gem 'msgpack', '~> 1.2', require: false
-  gem 'rdoc', ['~> 6.0', '< 6.4.0'], require: false, platforms: [:ruby]
+  gem 'rdoc', '~> 6.16', require: false, platforms: [:ruby]
   # requires native augeas headers/libs
   # gem 'ruby-augeas', require: false, platforms: [:ruby]
   # requires native ldap headers/libs