meta: update for Apple macOS branding

[imyller]

• URL: http://nodejs.org
• Browser version: all
• Operating system: all

Apple has updated their OS branding from OS X to macOS.

macOS (originally named "Mac OS X" until 2012 and then "OS X" until 2016)

Many places such as developer documentations, marketing materials and external sites like Wikipedia have started referring to macOS instead of OS X.

I'd suggest that Node.js site drops OS X references and uses following generic platform terms where possible:

• macOS
• Mac
• Macintosh

In my opinion, OS X can still be used if there is need to specify old OS version, for example: Apple OS X 10.9