cmd/golangorg, internal/history: add test case for Go 1.21.0

Go 1.21.0 is the first major Go release that explicitly includes the
trailing ".0" version component. Add a test case covering changes to
the history.Version type and the template that uses it.

For golang/go#57631.

Change-Id: I97581b650604c25b6996305ecf4ad9d883cd85ba
Reviewed-on: https://go-review.googlesource.com/c/website/+/497498
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

cmd/golangorg/testdata/release.txt

@@ -1,10 +1,16 @@
 GET https://go.dev/doc/devel/release
 header content-type == text/html; charset=utf-8
 trimbody contains
-	<h2 id="go1.19">go1.19 (released 2022-08-02)</h2>
+	<h2 id="go1.21.0">go1.21.0 (released 2023-08-08)</h2>
 	<p>
-	Go 1.19 is a major release of Go.
-	Read the <a href="/doc/go1.19">Go 1.19 Release Notes</a> for more information.
+	Go 1.21.0 is a major release of Go.
+	Read the <a href="/doc/go1.21">Go 1.21 Release Notes</a> for more information.
+	</p>
+trimbody contains
+	<h2 id="go1.20">go1.20 (released 2023-02-01)</h2>
+	<p>
+	Go 1.20 is a major release of Go.
+	Read the <a href="/doc/go1.20">Go 1.20 Release Notes</a> for more information.
 	</p>
 trimbody contains
 	<h2 id="go1.18">go1.18 (released 2022-03-15)</h2>