Clarify ruby version requirement

Right now the Readme says Ruby 1.9 (if you want well-ordered Hashes to be exported)

But we only run CI on ruby 2.3

I'd be in favor of officially dropping 1.9 support and only supporting supported rubies. I think that means 2.4, 2.5, and 2.6.

Agreed.

Defining a minimum version would also mean that we could clean up some old stuff. For example, we could generate hashes using Ruby 1.9+ syntax, instead of:

ruby-cldr/lib/cldr/export/data/plurals/rules.rb

Line 33 in def445f

    
           ruby = "{ :plural => { :keys => #{rule.keys.inspect}, :rule => #{rule.to_ruby} } }"

and only supporting supported rubies

At this point, I think we'd be fine dropping all the way up to Ruby 2.7 (exclusive): https://www.ruby-lang.org/en/downloads/branches/