doc/go1.21: finish last TODO for go/types.Package.GoVersion

Updates #58645.

Change-Id: I7352bf7f03c478d92ebd0b8345d676d5b2dfccb7
Reviewed-on: https://go-review.googlesource.com/c/go/+/513475
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

doc/go1.21.html

@@ -759,7 +759,8 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <dl id="go/types"><dt><a href="/pkg/go/types/">go/types</a></dt>
   <dd>
     <p><!-- https://go.dev/issue/61175, CL 507975 -->
-      TODO: <a href="https://go.dev/cl/507975">https://go.dev/cl/507975</a>: go/types: record Config.GoVersion for reporting in Package.GoVersion method; modified api/go1.21.txt
+      The new <a href="/pkg/go/types/#Package.GoVersion"><code>Package.GoVersion</code></a>
+      method returns the Go language version used to check the package.
     </p>
   </dd>
 </dl><!-- go/types -->