doc/go1.21: document macOS and Windows requirements

For #58645.
Fixes #57125.
Fixes #57003.
Fixes #57004.

Change-Id: I365929ca36aeb32e9ecd19f870e70869933ba6a0
Reviewed-on: https://go-review.googlesource.com/c/go/+/500239
Reviewed-by: Eli Bendersky <eliben@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

doc/go1.21.html

@@ -139,6 +139,22 @@ <h2 id="language">Changes to the language</h2>
 
 <h2 id="ports">Ports</h2>
 
+<h3 id="darwin">Darwin</h3>
+
+<p><!-- go.dev/issue/57125 -->
+  As <a href="go1.20#darwin">announced</a> in the Go 1.20 release notes,
+  Go 1.21 requires macOS 10.15 Catalina or later;
+  support for previous versions has been discontinued.
+</p>
+
+<h3 id="windows">Windows</h3>
+
+<p><!-- go.dev/issue/57003, go.dev/issue/57004 -->
+  As <a href="go1.20#windows">announced</a> in the Go 1.20 release notes,
+  Go 1.21 requires at least Windows 10 or Windows Server 2016;
+  support for previous versions has been discontinued.
+</p>
+
 <!-- CL 470695 -->
 <p>
   <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->