CMake: fix to support CMake 3.31.

CMake: fix to support CMake 3.31.

Expand the "still support old versions of CMake, for the benefit of
people with older versions, but avoid deprecation warnings/errors"
section to handle the deprecation of pre-3.10 versions by 3.31.

(cherry picked from commit 199bb821ba57097b8629a0d992779483b424e7a4)