fix: docs/v3/Gemfile & docs/v3/Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-RACK-569066

Author: snyk-bot

docs/v3/Gemfile

@@ -1,11 +1,11 @@
 source 'http://rubygems.org'
 
 gem 'json', '>= 2.3.0'
-gem 'middleman', '>= 3.4.1'
-gem 'middleman-autoprefixer', '>= 2.6.1'
+gem 'middleman', '>= 4.1.11'
+gem 'middleman-autoprefixer', '>= 2.10.1'
 gem 'middleman-gh-pages', '>= 0.0.3'
 gem 'middleman-livereload', '>= 3.4.6'
-gem 'middleman-syntax', '>= 2.0.0'
+gem 'middleman-syntax', '>= 2.1.0'
 gem 'mini_racer'
 gem 'nokogiri'
 gem 'rake', '>= 12.3.3'