internal/history: document Go 1.16.6 and Go 1.15.14

Change-Id: I2a585536147bd030f1bb333c05ffa45ef656bfcc
Reviewed-on: https://go-review.googlesource.com/c/website/+/334069
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>

internal/history/release.go

@@ -14,6 +14,20 @@ import "golang.org/x/website/internal/backport/html/template"
 //
 // The table is sorted by date, breaking ties with newer versions first.
 var Releases = []*Release{
+	{
+		Date: Date{2021, 7, 12}, Version: Version{1, 16, 6},
+		CustomSummary: `includes a security fix to the <code>crypto/tls</code> package,
+as well as bug fixes to the compiler, and the <code>net</code> and <code>net/http</code> packages. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.16.6+label%3ACherryPickApproved">Go
+1.16.6 milestone</a> on our issue tracker for details.`,
+	},
+	{
+		Date: Date{2021, 7, 12}, Version: Version{1, 15, 14},
+		CustomSummary: `includes a security fix to the <code>crypto/tls</code> package,
+as well as bug fixes to the linker, and the <code>net</code> package. See the
+<a href="https://github.com/golang/go/issues?q=milestone%3AGo1.15.14+label%3ACherryPickApproved">Go
+1.15.14 milestone</a> on our issue tracker for details.`,
+	},
 	{
 		Date: Date{2021, 6, 3}, Version: Version{1, 16, 5},
 		CustomSummary: `includes security fixes to the