java_requirement: support prompting users to install legacy Java casks

This enhances the Java requirement to support prompting the user
to install the correct legacy Java version via Cask for formulae
that don't yet work with the latest version of Java. Previously,
even if the formula had a strict requirement that a specific,
older version of Java be used, the messaging would tell the user to
`brew cask install java` (i.e. to install the latest version of Java),
which wouldn't actually satisfy the requirement.