crypto/tls: implement X25519MLKEM768

This makes three related changes that work particularly well together
and would require significant extra work to do separately: it replaces
X25519Kyber768Draft00 with X25519MLKEM768, it makes CurvePreferences
ordering crypto/tls-selected, and applies a preference to PQ key
exchange methods over key shares (to mitigate downgrades).

TestHandshakeServerUnsupportedKeyShare was removed because we are not
rejecting unsupported key shares anymore (nor do we select them, and
rejecting them actively is a MAY). It would have been nice to keep the
test to check we still continue successfully, but testClientHelloFailure
is broken in the face of any server-side behavior which requires writing
any other messages back to the client, or reading them.

Updates #69985
Fixes #69393

Change-Id: I58de76f5b8742a9bd4543fd7907c48e038507b19
Reviewed-on: https://go-review.googlesource.com/c/go/+/630775
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

key_schedule_test.go

@@ -6,7 +6,6 @@ package tls
 
 import (
 	"bytes"
-	"crypto/internal/fips140/mlkem"
 	"crypto/internal/fips140/tls13"
 	"crypto/sha256"
 	"encoding/hex"
@@ -118,21 +117,3 @@ func TestTrafficKey(t *testing.T) {
 		t.Errorf("cipherSuiteTLS13.trafficKey() gotIV = % x, want % x", gotIV, wantIV)
 	}
 }
-
-func TestKyberEncapsulate(t *testing.T) {
-	dk, err := mlkem.GenerateKey768()
-	if err != nil {
-		t.Fatal(err)
-	}
-	ct, ss, err := kyberEncapsulate(dk.EncapsulationKey().Bytes())
-	if err != nil {
-		t.Fatal(err)
-	}
-	dkSS, err := kyberDecapsulate(dk, ct)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if !bytes.Equal(ss, dkSS) {
-		t.Fatalf("got %x, want %x", ss, dkSS)
-	}
-}