The current HTML5 output uses FontAwesome 4.7
The far/fas style was added in FontAwesome 5 AFAIK
You can follow the discussion on upgrading AsciiDoctor to FontAwesome 5 on GitHub.
So the answer to your question is "no" considering out-of-the-box options.
There is always the chance of pass-through content (with the caveat mixing HTML with AsciiDoc and making the result less maintainable) or writing an extension of an inline macro that supports FA 5.