As of 3.31, cmake has deprecated support older than 3.10; while it still works, it complains, and probably won't work for too much longer. Per repology.org, Ubuntu 22.04 includes 3.22, Debian 11 has 3.25 available, CentOS Stream 8 has 3.26, and so on down the line. So this shouldn't be notable to anybody on a reasonably recent system.
This allows some minor cleanups, and removal of (probably scarcely tested) conditional support for older versions. Our usage of cmake's vars for finding Xrandr apparently already required 3.14 (addition of X11_Xrandr_FOUND etc), so anybody in the bottom half of this change was already was in bad shape anyway.