Update friendlier terminology in Gem::SafeYAML

[JuanitoFatas]

Use better terminology in Gem::SafeYAML, depends on ruby/psych#378.

Problem

These outdated terms could be offensive and harder to understand.

Description:

This Pull Request updates lib/rubygems/safe_yaml.rb.

Permitted is easier to understand and better terminology.

Original motivation, other community efforts examples:

• [ci skip] change all instances of blacklist and whitelist to denylist… rails/rails#33681
• deserialize_jsonapi not found on controller graphiti-api/graphiti#10
• Finish converting whitelist and blacklist references rails/rails#33718
• Use friendlier terminology in YAML.safe_load psych#378
• Use friendlier terminology in rubysocket.h ruby#2008
• Use friendlier terminology in _leaf_helpers.erb ruby#2009
• Use permitted in line_length.rb rubocop/rubocop#6464
• Update terminologies dtao/safe_yaml#93
• Introduce Request::ALLOWED_SCHEMES and deprecate existing constant rack/rack#1314
• Update terminology in http_status.rb rubocop/rubocop#6466
• Update terminology in rails/validation.rb rubocop/rubocop#6467
• Update terminology in BUILD_DETAIL.md rspec/rspec-core#2576
• Update terminology in BUILD_DETAIL.md rspec/rspec-expectations#1083
• Update terminology in BUILD_DETAIL.md rspec/rspec-mocks#1246
• Update terminology in BUILD_DETAIL.md rspec/rspec-support#356
• Use safelist(s), allowlist(s) where applicable flavorjones/loofah#158
• Use clearer terminologies pry/pry#1874
• Update terminology in test description from completion spec pry/pry#1875

---

Tasks:

• Describe the problem / feature
• Write tests
• Write code to solve the problem
• Get code review from coworkers / friends

I will abide by the code of conduct.

[bronzdoc]

👍 after https://github.com/dtao/safe_yaml/pull/93/files is merged

[hsbt]

This depends on ruby/psych#378. I will merge this after merging it.

[hsbt]

I merged ruby/psych#378 into ruby/ruby@db3101f. After updating ruby-head binary of Travis CI, I'm going to merge this.