I changed my mind and thought branching ERB.new in all libraries is too
hard. Code becomes too ugly. I increased the warn level to 2, and the
old initializer will be removed when Ruby 2.5 becomes EOL.
-S option of erb(1) stays in the same policy: will be removed at Ruby 2.7.
NEWS: note about the direction
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62612 b2dd03c8-39d4-4d8f-98ff-823fe69b080e