Update HTTParty

In order to update HTTParty, we'll need to adopt their new change.

HTTParty::Response to_s changed its implementation

In HTTParty 0.13.7, the Response#to_s was implemented in https://github.com/jnunemaker/httparty/blob/9d3dbc3852b294173d0d8309e59051491bf7d048/lib/httparty/response.rb#L65-L72 via:

    def method_missing(name, *args, &block)
      if parsed_response.respond_to?(name)
        parsed_response.send(name, *args, &block)
      elsif response.respond_to?(name)
        response.send(name, *args, &block)
      else
        super
      end

Now in 0.15.x it was changed this way in https://github.com/jnunemaker/httparty/blob/1a52a134c4aeef77668e56cbcf646d266d798389/lib/httparty/response.rb#L62-L68 to this:

    def to_s      
      if !response.nil? && !response.body.nil? && response.body.respond_to?(:to_s)
        response.body.to_s
      else 
        inspect
      end
    end

https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/57705954 is failing because WebHookService uses to_s, but doesn't expect the inspect implementation.