Drop Ruby < 2.3 support

Created by: pirj

Better viewed as https://github.com/rspec/rspec-core/pull/2787/files?diff=split&w=1

Sibling PRs:

• https://github.com/rspec/rspec-support/pull/436
• https://github.com/rspec/rspec-mocks/pull/1349
• https://github.com/rspec/rspec-expectations/pull/1231

TODO (not all):

• def self.ensure_example_groups_are_configured
• For some reason JRuby doesn't like our improved bundler setup
• echo "WARNING: Cucumber is skipped on JRuby on rspec-core due to" \
• lib/rspec/core/test_unit_assertions_adapter.rb
• [#] Mini[t|T]est - always Minitest
• ...