acme: update Directory URL to Let's Encrypt v2

This updates the default Directory URL to Let's Encrypt v2.
Their v1 has been deprecated and new account registrations
will be disabled in Nov 2019. See the following for details:
https://community.letsencrypt.org/t/end-of-life-plan-for-acmev1/88430

There's no good reason to keep the default URL pointing to v1
any longer.

Updates golang/go#21081

Change-Id: I34520e4dfb04a30d82a50559369bac819076d49c
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/200638
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

acme/acme.go

@@ -44,7 +44,7 @@ import (
 
 const (
 	// LetsEncryptURL is the Directory endpoint of Let's Encrypt CA.
-	LetsEncryptURL = "https://acme-v01.api.letsencrypt.org/directory"
+	LetsEncryptURL = "https://acme-v02.api.letsencrypt.org/directory"
 
 	// ALPNProto is the ALPN protocol name used by a CA server when validating
 	// tls-alpn-01 challenges.