crypto/internal/mlkem768: unexport encryptionKey.A

crypto/internal/mlkem768: unexport encryptionKey.A

Change-Id: I001e48dbb476e590cd84194f386bf5ea4cc1a1b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/621976
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>