Rename OS X as macOS on our website

https://en.wikipedia.org/wiki/MacOS: “In 2016, Apple rebranded OS X to macOS, adopting the nomenclature that it uses for their other operating systems, iOS, watchOS, and tvOS.”

It’s not going to be very exciting but the longer we wait the more occurences we’ll have to fix and the more translation strings we might break.

Feature Branch: https://github.com/cbrownstein/tails/tree/doc/12490-rename-os-x

Related issues

Blocks #13423 (closed)

Original created by @sajolida on 12490 (Redmine)

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