crypto/mlkem: merge mlkem768.go and mlkem1024.go to improve godoc

crypto/mlkem: merge mlkem768.go and mlkem1024.go to improve godoc

The constants appeared badly ordered and grouped in the godoc before

    const (
        CiphertextSize1024 = 1568
        EncapsulationKeySize1024 = 1568
    )
    const (
        SharedKeySize = 32
        SeedSize = 64
        CiphertextSize768 = 1088
        EncapsulationKeySize768 = 1184
    )

while now they are a single group with the good size first

    const (
        SharedKeySize = 32
        SeedSize = 64
        CiphertextSize768 = 1088
        EncapsulationKeySize768 = 1184
        CiphertextSize1024 = 1568
        EncapsulationKeySize1024 = 1568
    )

No code changes.

Change-Id: I6a6a4656961b1e8c8bca3992aafa33e0575af8a2
Reviewed-on: https://go-review.googlesource.com/c/go/+/640997
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>