List puppet as runtime dependency

puppe is a runtime dependency:

```
$ git grep "require 'puppet/"
lib/puppet-strings/yard/parsers/puppet/parser.rb:require 'puppet/pops'
lib/puppet-strings/yard/parsers/puppet/statement.rb:require 'puppet/pops'
lib/puppet-strings/yard/util.rb:require 'puppet/util'
lib/puppet/application/strings.rb:require 'puppet/application/face_base'
lib/puppet/face/strings.rb:require 'puppet/face'
lib/puppet/feature/rgen.rb:require 'puppet/util/feature'
lib/puppet/feature/yard.rb:require 'puppet/util/feature'
spec/spec_helper.rb:require 'puppet/version'
```

puppet-strings.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   ]
   s.files = Dir['CHANGELOG.md', 'README.md', 'LICENSE', 'lib/**/*', 'exe/**/*']
 
+  s.add_runtime_dependency 'puppet', '>= 7.0.0'
   s.add_runtime_dependency 'rgen', '~> 0.9'
   s.add_runtime_dependency 'yard', '~> 0.9'
-  s.requirements << 'puppet, >= 7.0.0'
 end