CMake: Bump maximum policy version to 3.31

CMake: Bump maximum policy version to 3.31

With CMake 3.31, there were a few warnings from
CMP0177 "install() DESTINATION paths are normalized".
These occurred because the install(FILES) command in
my_install_man_lang() is called with a DESTINATION path
that contains two consecutive slashes, for example,
"share/man//man1". Such a path is for the English man pages.
With translated man pages, the language code goes between
the slashes. The warning was probably triggered because the
extra slash gets removed by the normalization.

(cherry picked from commit 61feaf681bd793dc5c919732b44bca7dcf2ed1b8)