EAPI usage guidelines for base system packages

Gentoo council says we need to maintain an upgrade path for 1 year.
Essentially, this means users must be able to upgrade to a usable version of
Portage.

Portage and its dependencies (both direct and indirect) must utilize an EAPI
that is compatible with the stable version of Portage as of 1 year ago.

This also includes packages that are part of @system, as defined by any
stable profiles. Optional dependencies of @system (such as X11) are excluded.