Update documented minimum supported OSes

Drop support for macOS 10.13, macOS 10.14, Android 4.1, Android 4.2,
Windows 7, Windows 8, Windows 8.1.

Change-Id: I39267898e9c6366ddfb35430fa0245cd5498eb47
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/841657
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
2 files changed