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'
```

Author: bastelfreak

puppet-strings.gemspec

@@ -24,5 +24,5 @@ Gem::Specification.new do |s|
 
   s.add_runtime_dependency 'rgen', '~> 0.9'
   s.add_runtime_dependency 'yard', '~> 0.9'
-  s.requirements << 'puppet, >= 7.0.0'
+  s.add_runtime_dependency 'puppet', '>= 7.0.0'
 end