macOS: Drop support for macOS 10.9 and older from official binaries

Apple ended its support for macOS 10.9 in Sep 2016. I propose for CMake 3.19, we drop support for 10.9 and older in the official binaries (which are currently targeting 10.7). This is a 4 year window where CMake supports an EOL release with the latest releases. This cutoff was determined as of now based on the testing support being done for CMake itself. Users of the older releases can compile from source (with "best effort, but untested" upstream support) or use older CMake releases.

Table of dates: https://en.wikipedia.org/wiki/MacOS_version_history#Releases

In the future, I propose that we drop support for releases of macOS that Apple has stopped supporting 4 years prior (which should be enough time to either migrate or not really be able to compile/ship code that would require a newer CMake in any case).

Cc: @brad.king