Go 1.14 will drop support for macOS 10.10, see #23011

This reverts CL 125304

Updates #26475
Updates #26513

Change-Id: Ia13eef30f22d67103f7ae45424124fbb116e1261
Reviewed-on: https://go-review.googlesource.com/c/go/+/214057
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>