Update rdoc requirement from ~> 6.14 to ~> 7.0

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@v6.14.0...v7.0.1)

---
updated-dependencies:
- dependency-name: rdoc
  dependency-version: 7.0.1
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

slacken.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |gem|
 
   gem.add_development_dependency 'bundler'
   gem.add_development_dependency 'rake', '~> 13.2'
-  gem.add_development_dependency 'rdoc', '~> 6.14'
+  gem.add_development_dependency 'rdoc', '~> 7.0'
   gem.add_development_dependency 'rspec', '~> 3.0'
   gem.add_development_dependency 'rubygems-tasks', '~> 0.2'
   gem.add_development_dependency 'unindent', '~> 1.0'