sys: Reference 15.0/16.0 for 32-bit kernel deprecation

[emaste]

Summary

In 15.x armv7 is the only supported 32-bit kernel, and it may be removed
from 16.0.  In 14.x however 32-bit powerpc and i386 kernels are still
supported in 14.0, and we don't want to imply they will persist until
16.0.

It doesn't seem worth having per-arch messages, for for 14.x just report
15.0/16.0.

[emaste]

(for direct commit to stable/14)

[emaste]

Just reverted instead, "may be" removed in 15.0 is good enough.