refine the KMS admin API (#8943)

This commit removes the `Update` functionality
from the admin API. While this is technically
a breaking change I think this will not cause
any harm because:
 - The KMS admin API is not complete, yet.
   At the moment only the status can be fetched.
 - The `mc` integration hasn't been merged yet.
   So no `mc` client could have used this API
   in the past.

The `Update`/`Rewrap` status is not useful anymore.
It provided a way to migrate from one master key version
to another. However, KES does not support the concept of
key versions. Instead, key migration should be implemented
as migration from one master key to another.

Basically, the `Update` functionality has been implemented just
for Vault.

cmd/admin-handlers.go

@@ -1270,20 +1270,7 @@ func (a adminAPIHandlers) KMSKeyStatusHandler(w http.ResponseWriter, r *http.Req
 		return
 	}
 
-	// 2. Check whether we can update / re-wrap the sealed key.
-	sealedKey, err = GlobalKMS.UpdateKey(keyID, sealedKey, kmsContext)
-	if err != nil {
-		response.UpdateErr = err.Error()
-		resp, err := json.Marshal(response)
-		if err != nil {
-			writeCustomErrorResponseJSON(ctx, w, errorCodes.ToAPIErr(ErrInternalError), err.Error(), r.URL)
-			return
-		}
-		writeSuccessResponseJSON(w, resp)
-		return
-	}
-
-	// 3. Verify that we can indeed decrypt the (encrypted) key
+	// 2. Verify that we can indeed decrypt the (encrypted) key
 	decryptedKey, err := GlobalKMS.UnsealKey(keyID, sealedKey, kmsContext)
 	if err != nil {
 		response.DecryptionErr = err.Error()
@@ -1296,7 +1283,7 @@ func (a adminAPIHandlers) KMSKeyStatusHandler(w http.ResponseWriter, r *http.Req
 		return
 	}
 
-	// 4. Compare generated key with decrypted key
+	// 3. Compare generated key with decrypted key
 	if subtle.ConstantTimeCompare(key[:], decryptedKey[:]) != 1 {
 		response.DecryptionErr = "The generated and the decrypted data key do not match"
 		resp, err := json.Marshal(response)

pkg/madmin/kms-commands.go

@@ -57,6 +57,5 @@ func (adm *AdminClient) GetKeyStatus(keyID string) (*KMSKeyStatus, error) {
 type KMSKeyStatus struct {
 	KeyID         string `json:"key-id"`
 	EncryptionErr string `json:"encryption-error,omitempty"` // An empty error == success
-	UpdateErr     string `json:"update-error,omitempty"`     // An empty error == success
 	DecryptionErr string `json:"decryption-error,omitempty"` // An empty error == success
 }