internal/history: document Go 1.21.5 and 1.20.12 releases

Change-Id: I63f25161b6a65be28c85faba67a0c8bf87f63dc8
Reviewed-on: https://go-review.googlesource.com/c/website/+/547535
Commit-Queue: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

internal/history/release.go

@@ -14,6 +14,26 @@ import "html/template"
 //
 // The table is sorted by date, breaking ties with newer versions first.
 var Releases = []*Release{
+	{
+		Date: Date{2023, 12, 5}, Version: Version{1, 21, 5},
+		Security: &FixSummary{
+			Components: []template.HTML{"the <code>go</code> command"},
+			Packages:   []string{"net/http", "path/filepath"},
+		},
+		Bug: &FixSummary{
+			Components: []template.HTML{"the compiler", "the <code>go</code> command", "the runtime"},
+			Packages:   []string{"crypto/rand", "net", "os", "syscall"},
+		},
+	},
+	{
+		Date: Date{2023, 12, 5}, Version: Version{1, 20, 12},
+		Security: &FixSummary{
+			Components: []template.HTML{"the <code>go</code> command"},
+			Packages:   []string{"net/http", "path/filepath"},
+		},
+		Bug: &FixSummary{
+			Components: []template.HTML{"the compiler", "the <code>go</code> command"}},
+	},
 	{
 		Date: Date{2023, 11, 7}, Version: Version{1, 21, 4},
 		Security: &FixSummary{Packages: []string{"path/filepath"}},