[ruby/irb] Drop Ruby 2.6 support

(https://github.com/ruby/irb/pull/555)

* Remove all Ruby 2.6 support

* Drop Ruby 2.6 specific testing conditions

* Only run Ruby 2.7+ on CI

* Bump Ruby requirement to 2.7+

3f714b616c

test/irb/test_cmd.rb

@@ -855,9 +855,6 @@ module TestIRB
     end
 
     def test_edit_with_constant
-      # const_source_location is supported after Ruby 2.7
-      omit if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.7.0')
-
       out, err = execute_lines(
         "edit IRB::Irb"
       )