internal/history: add Go 1.20, remove release notes draft notice

Fixes golang/go#54202.

Change-Id: I6043b8af0e3de7e46e5ab0e9dafe9c34cbd18236
Reviewed-on: https://go-review.googlesource.com/c/website/+/464175
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

_content/doc/go1.20.html

@@ -14,13 +14,13 @@
   main ul li { margin: 0.5em 0; }
 </style>
 
-<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.20</h2>
+<h2 id="introduction">Introduction to Go 1.20</h2>
 
 <p>
-  <strong>
-    Go 1.20 is not yet released. These are work-in-progress
-    release notes. Go 1.20 is expected to be released in February 2023.
-  </strong>
+  The latest Go release, version 1.20, arrives six months after <a href="/doc/go1.19">Go 1.19</a>.
+  Most of its changes are in the implementation of the toolchain, runtime, and libraries.
+  As always, the release maintains the Go 1 <a href="/doc/go1compat">promise of compatibility</a>.
+  We expect almost all Go programs to continue to compile and run as before.
 </p>
 
 <h2 id="language">Changes to the language</h2>

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, 2, 1}, Version: Version{1, 20, 0},
+	},
 	{
 		Date: Date{2023, 1, 10}, Version: Version{1, 19, 5},
 		Bug: &FixSummary{