Move unicode_requirements() to system module

Problem/Motivation

unicode_requirements() is just a helper for system_requirements().

Proposed resolution

Move or copy the logic to system_requirements().

Deprecate unicode_requirements() for removal in 9.x, although in this case we might be able to get away with removing it in a minor since it should never, ever be called.

Remaining tasks

User interface changes

API changes

Data model changes