[rubygems/rubygems] Add new default gems to setup_spec exempts.

- error_highlight was introduced at e946049665
  orriginally as error_squiggle later renamed at 9438c99590

- ruby2_keywords was introduced as a placeholder gem only at 21d2463fbc

https://github.com/rubygems/rubygems/commit/c9ebe7c7d2

spec/bundler/runtime/setup_spec.rb

@@ -1299,6 +1299,8 @@ end
         exempts << "pathname" if Gem.ruby_version >= Gem::Version.new("3.0")
         exempts << "set" unless Gem.rubygems_version >= Gem::Version.new("3.2.6")
         exempts << "tsort" if Gem.ruby_version >= Gem::Version.new("3.0")
+        exempts << "error_highlight" # added in Ruby 3.1 as a default gem
+        exempts << "ruby2_keywords" # added in Ruby 3.1 as a default gem
         exempts
       end