build: increase macOS minimum supported version

Starting with V8 6.8, it will not be possible to build with macOS 10.10
and below. Those versions are already EoL.

PR-URL: https://github.com/nodejs/node/pull/21883
Refs: https://github.com/nodejs/build/issues/1358


Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>