doc/go1.14: preannounce dropping macOS 10.11 support

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>

doc/go1.14.html

@@ -56,6 +56,11 @@ TODO
 
 <h3 id="darwin">Darwin</h3>
 
+<p>
+  Go 1.14 is the last release that will run on macOS 10.11 El Capitan.
+  Go 1.15 will require macOS 10.12 Sierra or later.
+</p>
+
 <p><!-- golang.org/issue/34749 -->
   Go 1.14 is the last Go release to support 32-bit binaries on
   macOS (the <code>darwin/386</code> port). They are no longer