doc/install/gccgo: update for GCC 12 and 13

Fixes golang/go#59143

Change-Id: I15f04244f3eda0292c6895fd2c2e816c86183ac7
Reviewed-on: https://go-review.googlesource.com/c/website/+/477875
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: ianlancetaylor

_content/doc/install/gccgo.html

@@ -86,6 +86,12 @@ <h2 id="Releases">Releases</h2>
 release.
 </p>
 
+<p>
+The GCC 12 and 13 releases include a complete implementation of the Go
+1.18 standard library. However, GCC does not yet include support for
+generics.
+</p>
+
 <h2 id="Source_code">Source code</h2>
 
 <p>