Deprecate Ruby 2.3 support

[flavorjones]

As noted at https://www.ruby-lang.org/en/news/2019/03/31/support-of-ruby-2-3-has-ended/ upstream support for Ruby 2.3 ended on March 31, 2019.

This issue is to formally remove Ruby 2.3 support from Nokogiri, including;

• update README and gemspec to indicate this change
• remove Ruby 2.3 from CI pipelines
• remove Ruby 2.3 from windows cross-compiled gem (.cross_rubies)

Marking this for a 1.11.0 release to coincide with a minor version bump (not a patchlevel version bump).

[flavorjones]

I want to note that deprecating Ruby 2.3 support should also deprecate JRuby 9.1 support.

[flavorjones]

Merged #1917 and made a few other changes. Closing.