Remove useless TARGET_RUBY_VERSION env var

[koic]

This PR removes useless TARGET_RUBY_VERSION env var and uses PARSER_ENGINE env var instead.

In fact, the ruby_version in shared_context is replaced with the value required by PARSER_ENGINE:

rubocop-ast/spec/spec_helper.rb

Lines 22 to 60 in 49b135b

	RSpec.shared_context 'ruby 2.3', :ruby23 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 2.3 }
	end
	RSpec.shared_context 'ruby 2.4', :ruby24 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 2.4 }
	end
	RSpec.shared_context 'ruby 2.5', :ruby25 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 2.5 }
	end
	RSpec.shared_context 'ruby 2.6', :ruby26 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 2.6 }
	end
	RSpec.shared_context 'ruby 2.7', :ruby27 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 2.7 }
	end
	RSpec.shared_context 'ruby 3.0', :ruby30 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 3.0 }
	end
	RSpec.shared_context 'ruby 3.1', :ruby31 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 3.1 }
	end
	RSpec.shared_context 'ruby 3.2', :ruby32 do
	# Prism supports parsing Ruby 3.3+.
	let(:ruby_version) { ENV['PARSER_ENGINE'] == 'parser_prism' ? 3.3 : 3.2 }
	end