Replace deprecated elliptic.Marshal (#26800)

In PR #26786, the Go version for golangci-lint is bumped to 1.21. This
causes the following error:

```
models/migrations/v1_16/v210.go:132:23: SA1019: elliptic.Marshal has been deprecated since Go 1.21: for ECDH, use the crypto/ecdh package. This function returns an encoding equivalent to that of PublicKey.Bytes in crypto/ecdh. (staticcheck)
                                        PublicKey:       elliptic.Marshal(elliptic.P256(), parsed.PubKey.X, parsed.PubKey.Y),
```

The change now uses [func (*PublicKey)
ECDH](https://pkg.go.dev/crypto/ecdsa#PublicKey.ECDH), which is added in
Go 1.20.

models/migrations/v1_16/v210.go

@@ -4,7 +4,6 @@
 package v1_16 //nolint
 
 import (
-	"crypto/elliptic"
 	"encoding/base32"
 	"fmt"
 	"strings"
@@ -123,13 +122,17 @@ func RemigrateU2FCredentials(x *xorm.Engine) error {
 				if err != nil {
 					continue
 				}
+				pubKey, err := parsed.PubKey.ECDH()
+				if err != nil {
+					continue
+				}
 				remigrated := &webauthnCredential{
 					ID:              reg.ID,
 					Name:            reg.Name,
 					LowerName:       strings.ToLower(reg.Name),
 					UserID:          reg.UserID,
 					CredentialID:    base32.HexEncoding.EncodeToString(parsed.KeyHandle),
-					PublicKey:       elliptic.Marshal(elliptic.P256(), parsed.PubKey.X, parsed.PubKey.Y),
+					PublicKey:       pubKey.Bytes(),
 					AttestationType: "fido-u2f",
 					AAGUID:          []byte{},
 					SignCount:       reg.Counter,