doc/go1.20: preannounce dropping macOS 10.13 and 10.14 support

For #23011.

Change-Id: I386920928a98403180098f1da5ea7696a239210e
Reviewed-on: https://go-review.googlesource.com/c/go/+/454957
Auto-Submit: Heschi Kreinick <heschi@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Heschi Kreinick <heschi@google.com>
diff --git a/doc/go1.20.html b/doc/go1.20.html
index 177b27e..26f65f2 100644
--- a/doc/go1.20.html
+++ b/doc/go1.20.html
@@ -67,6 +67,13 @@
 
 <h2 id="ports">Ports</h2>
 
+<h3 id="darwin">Darwin and iOS</h3>
+
+<p><!-- golang.org/issue/23011 -->
+  Go 1.20 is the last release that will run on macOS 10.13 High Sierra or 10.14 Mojave.
+  Go 1.21 will require macOS 10.15 Catalina or later.
+</p>
+
 <h3 id="freebsd-riscv">FreeBSD/RISC-V</h3>
 
 <p><!-- https://go.dev/issue/53466 -->