Update Gem.gemspec

[v20100v]

Gem has_rdoc, which is deprecated in the latest RubyGems API.

Cause a warning message after doing bundle install or update.

[v20100v]

I have another remarks.

Is it possible to create a new release version for Jekyll menu (0.6.1) ?

To use Jekyll-Menu in a Ruby 3.2.2 environment, we need to install it with this ugly hack :

# Gemfile
(...)
group :jekyll_plugins do
   gem "jekyll-menus", :git => "https://github.com/forestryio/jekyll-menus.git"
end

If we precise a rule version ot install Jekyll-menus like bellow, it does not install the last content of git repository.

spec.add_runtime_dependency "jekyll-menus", "~> 0.6"

The last tagged version does not match the code status of the git repository. In other words, the master branch does not have a release tag.

Thanks in advance ^^.