Refactor docs raketask to use next unless

While adding the docs raketask to Omnibus in
https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/5275,
a rubocop offense suggested to use next. This ports the change
to the GitLab raketask as well.

lib/tasks/gitlab/docs/redirect.rake

@@ -112,14 +112,13 @@ namespace :gitlab do
         # The remove_date of redirects.yaml should be nine months in the future.
         # To not be confused with the remove_date of the Markdown page.
         #
-        if remove_date < today
-          File.delete(filename) if File.exist?(filename)
+        next unless remove_date < today
 
+        File.delete(filename) if File.exist?(filename)
         puts "  - from: #{old_path}"
         puts "    to: #{new_path(frontmatter['redirect_to'], filename)}"
         puts "    remove_date: #{remove_date >> 9}"
       end
     end
   end
-  end
 end