Add string encoding IBM720 alias CP720 since 3.0.0

[ima1zumi]

Ruby 3.0 から追加された Encoding::CP720 の説明を追加します。

irb(main):002:0> RUBY_VERSION
=> "3.0.2"
irb(main):001:0> 'ب'.encode(Encoding::CP720)
=> "\xA0"

irb(main):001:0> RUBY_VERSION
=> "2.7.4"
irb(main):002:0> 'ب'.encode(Encoding::CP720)
Traceback (most recent call last):
        4: from /Users/mi/.asdf/installs/ruby/2.7.4/bin/irb:23:in `<main>'
        3: from /Users/mi/.asdf/installs/ruby/2.7.4/bin/irb:23:in `load'
        2: from /Users/mi/.asdf/installs/ruby/2.7.4/lib/ruby/gems/2.7.0/gems/irb-1.2.6/exe/irb:11:in `<top (required)>'
        1: from (irb):2
NameError (uninitialized constant Encoding::CP720)
Did you mean?  Encoding::CP1250

ref: #2458
ref: https://bugs.ruby-lang.org/issues/16233
ref: https://en.wikipedia.org/wiki/Code_page_720