RELNOTES: Note the deprecation of 32-bit platforms for 15.0.

This draws a line in the sand of removing support for 32-bit worlds
and kernels aside from COMPAT_FREEBSD32 and lib32 support. The
project may choose to alter this approach when 15.0 is released by
extending some level of 32-bit support in 15.0 or later.

In particular, at present there is a fair bit of interest in retaining
armv7 support in 15.0 at least.

Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D41164