crypto/hpke: apply fips140.WithoutEnforcement to ML-KEM+X25519 hybrid

crypto/hpke: apply fips140.WithoutEnforcement to ML-KEM+X25519 hybrid

Since it uses an Approved KEM (ML-KEM), the overall hybrid KEM is
Approved, even if X25519 is not.

Updates #70514
Updates #74630

Change-Id: I2bb60c36fcf570baa3c389e2daa3698e6a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/728505
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>