internal/history: add Go 1.21, remove release notes draft notice

Also, update the golangorg deployment to use the Go 1.21 release branch.

Fixes golang/go#58645.

Change-Id: Ic83bbd34da083fd0358e4e345fdb02781cab43d6
Reviewed-on: https://go-review.googlesource.com/c/website/+/517035
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/doc/go1.21.html

@@ -14,14 +14,7 @@
   main ul li { margin: 0.5em 0; }
 </style>
 
-<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.21</h2>
-
-<p>
-  <strong>
-    Go 1.21 is not yet released. These are work-in-progress
-    release notes. Go 1.21 is expected to be released in August 2023.
-  </strong>
-</p>
+<h2 id="introduction">Introduction to Go 1.21</h2>
 
 <p>
   The latest Go release, version 1.21, arrives six months after <a href="/doc/go1.20">Go 1.20</a>.

cmd/golangorg/cloudbuild.yaml

@@ -4,7 +4,7 @@
 
 steps:
   - name: gcr.io/cloud-builders/git
-    args: ["clone", "--branch=release-branch.go1.20", "--depth=1", "https://go.googlesource.com/go", "_gotmp"]
+    args: ["clone", "--branch=release-branch.go1.21", "--depth=1", "https://go.googlesource.com/go", "_gotmp"]
   - name: gcr.io/cloud-builders/git
     args: ["archive", "--format=zip", "--output=../_goroot.zip", "HEAD"]
     dir: _gotmp

internal/history/release.go

@@ -14,6 +14,9 @@ import "html/template"
 //
 // The table is sorted by date, breaking ties with newer versions first.
 var Releases = []*Release{
+	{
+		Date: Date{2023, 8, 8}, Version: Version{1, 21, 0},
+	},
 	{
 		Date: Date{2023, 8, 1}, Version: Version{1, 20, 7},
 		Security: &FixSummary{Quantifier: "a", Packages: []string{"crypto/tls"}},