The list `rbenv install -l` is too long, show latest stable releases by default

Add artichoke-dev to install the latest Artichoke nightly build

Allow to install the latest official Artichoke nightly build from https://github.com/artichoke/nightly/releases

CVE-2020-1967, bump openssl to 1.1.1g

mark ruby 2.5 as security maintenance

s/all built-in definitions/all local definitions/

Update to OpenSSL 1.1.1f for Ruby 2.5+

ruby-build 20200401

Add definition for JRuby 9.2.11.1

add definition 2.4.10, 2.5.8, 2.6.6, 2.7.1 and ruby 2.4 EOL

JRuby 9.2.11.1 has been released.
https://www.jruby.org/2020/03/25/jruby-9-2-11-1

"available" implies ruby-build will be checking online sources but
actually ruby-build doesn't do that. "local definitions" may be
prefered.

Pointed out by jasonkarns.

Suggested by mislav: https://github.com/rbenv/ruby-build/pull/1402#discussion_r389060742

> I wonder should we substitute "built-in" with something like "all
> available definitions". Since ruby-build will additionally pick up
> definitions found under the paths listed in RUBY_BUILD_DEFINITIONS,
> these definitions will get listed even if they are not "built-in".
> Therefore, "built-in" feels like a misnomer here.

Add update-mri and update-jruby scripts

This PR adds update-cruby and update-jruby scripts.
They are provided to update MRI and JRuby definition files.

These scripts are based on script/update-truffleruby.
https://github.com/rbenv/ruby-build/blob/v20200224/script/update-truffleruby

Added rbx-4.*

as it is not necessary. Pointed out by @mislav in review.

This is out of scope of this PR.

Pointed out by @mislav in review

for each Ruby implementations like the following.

    2.4.9
    2.5.7
    2.6.5
    2.7.0
    jruby-9.2.9.0
    maglev-1.0.0
    mruby-2.1.0
    rbx-3.107
    truffleruby-19.3.1

it is not actually EoL'ed.  `warn_unsupported` indicates
that the versions are nearing EoL.

Add definition for JRuby 9.2.11.0

JRuby 9.2.11.0 has been released.
https://www.jruby.org/2020/03/02/jruby-9-2-11-0

Add definition for JRuby 9.2.10.0

JRuby 9.2.10.0 has been released.
https://www.jruby.org/2020/02/18/jruby-9-2-10-0.html