crypto/hpke: use new gcm.NewGCMForHPKE for FIPS 140-3 compliance

crypto/hpke: use new gcm.NewGCMForHPKE for FIPS 140-3 compliance

It does the exact same thing, but we can document it as an allowed and
enforced nonce scheme in the Security Policy.

Change-Id: I9d95ba53354e5c8112cde24101570d4b6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/728503
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>