crypto/internal/mlkem768: add -768 suffix to all exported identifiers

crypto/internal/mlkem768: add -768 suffix to all exported identifiers

In preparation for introducing ML-KEM-1024.

Aside from the constants at the top, all other changes were automated.

Change-Id: I0fafce9a776c7b0b9179be1c858709cabf60e80f
Reviewed-on: https://go-review.googlesource.com/c/go/+/621981
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>