docs: rebrand OS X and OSX references to macOS (#1689)
## Situation Several website pages on https://docs.npmjs.com, such as https://docs.npmjs.com/downloading-and-installing-node-js-and-npm refer to the operating system "OS X" and "OSX". The last version of OS X was OS X 10.11 (El Capitan), released on Sep 30, 2015. Apple rebranded to macOS with the macOS 10.12 (Sierra) release on Sep 30, 2016. ## Change Rename "OS X" and "OSX" to "macOS". ## References https://en.wikipedia.org/wiki/MacOS_version_history https://www.apple.com/macos/ Related to https://github.com/npm/documentation/issues/1459 Follows on from https://github.com/npm/cli/pull/8371 which only changed the branding in the CLI v11 documentation content, such as https://docs.npmjs.com/cli/v11/configuring-npm/install, not in non-CLI website content.