doc/go1.16: preannounce dropping macOS 10.12 support

Go 1.16 will be the last to support macOS 10.12 Sierra.
Go 1.17 will require macOS 10.13 High Sierra.

For #23011.

Change-Id: I80052bdde4d9f1c5d71b67b85f65fb0b40856750
Reviewed-on: https://go-review.googlesource.com/c/go/+/275299
Trust: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: dmitshur

doc/go1.16.html

@@ -58,6 +58,11 @@ <h3 id="darwin">Darwin and iOS</h3>
   the <code>ios</code> build tag set.
 </p>
 
+<p><!-- golang.org/issue/23011 -->
+  Go 1.16 is the last release that will run on macOS 10.12 Sierra.
+  Go 1.17 will require macOS 10.13 High Sierra or later.
+</p>
+
 <h3 id="netbsd">NetBSD</h3>
 
 <p><!-- golang.org/issue/30824 -->