More rubies on CI, deprecating ruby 2.3 due to test failure

[mathieujobin]

ruby 2.3 now raise an ArgumentError while running tests

 ........E
===============================================================================
Error: test_slow_connect(TestWEBrickSSLServer): ArgumentError: wrong number of arguments (given 3, expected 1..2)
/opt/hostedtoolcache/Ruby/2.3.8/x64/lib/ruby/2.3.0/timeout.rb:73:in `timeout'
/home/runner/work/webrick/webrick/test/lib/envutil.rb:72:in `timeout'
/home/runner/work/webrick/webrick/test/webrick/test_ssl_server.rb:58:in `test_slow_connect'
     55:       :SSLEnable => true,
     56:       :SSLCertName => "/C=JP/O=www.ruby-lang.org/CN=Ruby",
     57:     }
  => 58:     EnvUtil.timeout(10) do
     59:       TestWEBrick.start_server(Echo, config) do |server, addr, port, log|
     60:         outer = TCPSocket.new(addr, port)
     61:         inner = TCPSocket.new(addr, port)
===============================================================================

making 2.4 the minimum and including 3.0 as base ruby

[hsbt]

👍