Let Regexp.new accept any options value

[soutaro]

Regexp.new accepts any value as options while the doc says it accepts Integer | nil | false. I found a code passing true and it cannot be rejected by a type checker, because the RDoc of Regexp.new shows it as an example.

This change will be reverted once Ruby 3.2 is out. The unexpected options value will be rejected by a runtime error on 3.2.

(See also #1040)