The name was "Mac OS X" during versions 10.0 to 10.7.
It was renamed to "OS X" starting from 10.8 until 10.11.
The current name is "macOS" starting with 10.12. [1]

Previous changes (e.g., CL 47252) updated "Mac OS X" to macOS
in some places, but not everywhere. This CL updates remaining
instances for consistency.

Only the pages that display current information were updated;
historical pages such as release notes for older Go releases,
past articles, blog posts, etc., were left in original form.

Rename the "#osx" anchor to "#macos" on /doc/install page,
along with the single reference to it on the same page.
Add an empty div with id="osx" to not break old links.

Update minimum macOS version from 10.8 to 10.10 per #23122.

[1]: https://en.wikipedia.org/wiki/macOS#History

Updates #23122.

Change-Id: I69fe4b85e83265b9d99f447e3cc5230dde094869
Reviewed-on: https://go-review.googlesource.com/119855Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>