fix: require cgi/escape to avoid Ruby 4 CGI warning

[masterkain]

Summary

• replace require "cgi" with require "cgi/escape" in lib/oauth2.rb
• keep escape/unescape support while avoiding Ruby 4 warning about removed cgi stdlib

Reproduction

ruby -w -Ilib -e 'require "oauth2"'

Before this change on Ruby 4, warning includes:

CGI library is removed from Ruby 4.0

Verification

ruby -w -Ilib -e 'require "oauth2"; puts OAuth2::Version::VERSION'
bundle exec rspec spec/oauth2/strategy/auth_code_spec.rb spec/oauth2/strategy/implicit_spec.rb spec/oauth2/access_token_spec.rb

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d81a98b) to head (2babd4b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #700   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          567       567           
  Branches       174       174           
=========================================
  Hits           567       567           

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