doc/go1.21: mention upgrade to Unicode 15.0.0

For #48621
For #55079

Change-Id: I279b4fbf07d2fa9de9577559a4e17a57f5692aa3
Reviewed-on: https://go-review.googlesource.com/c/go/+/499618
Reviewed-by: Eli Bendersky <eliben@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Ian Lance Taylor <iant@google.com>

doc/go1.21.html

@@ -284,10 +284,6 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   There are also various performance improvements, not enumerated here.
 </p>
 
-<p><!-- CL 456837 -->
-  TODO: <a href="https://go.dev/cl/456837">https://go.dev/cl/456837</a>: all: upgrade Unicode from 13.0.0 to 15.0.0; modified api/except.txt, api/next/55079.txt
-</p>
-
 <p>
   TODO: complete this section
 </p>
@@ -992,6 +988,15 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- testing/slogtest -->
 
+<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
+  <dd>
+    <p><!-- CL 456837 -->
+      The <a href="/pkg/unicode/"><code>unicode</code></a> package and
+      associated support throughout the system has been upgraded to
+      <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
+    </p>
+</dl><!-- unicode -->
+
 <!-- proposals for x repos that don't need to be mentioned here but
      are picked up by the relnote tool. -->
 <!-- https://go.dev/issue/54232 -->