Go 1.14 will be the last to support macOS 10.11.
Go 1.15 will require macOS 10.12 (Sierra).

Updates #23011

Change-Id: I8fff555e5b8fffe088e7e960e77fac9558cb74e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/210137Reviewed-by: Andrew Bonventre <andybons@golang.org>