BigInteger: fix deprecation notice

phpseclib/Math/BigInteger.php

@@ -443,6 +443,7 @@ class BigInteger implements \JsonSerializable
      *
      * Will be called, automatically, when json_encode() is called on a BigInteger object.
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         $result = ['hex' => $this->toHex(true)];

phpseclib/Math/BigInteger/Engines/Engine.php

@@ -376,6 +376,7 @@ abstract class Engine implements \JsonSerializable
      *
      * Will be called, automatically, when json_encode() is called on a BigInteger object.
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         $result = ['hex' => $this->toHex(true)];

phpseclib/Math/Common/FiniteField/Integer.php

@@ -31,6 +31,7 @@ abstract class Integer implements \JsonSerializable
      * PHP Serialize isn't supported because unserializing would require the factory be
      * serialized as well and that just sounds like too much
      */
+    #[\ReturnTypeWillChange]
     public function jsonSerialize()
     {
         return ['hex' => $this->toHex(true)];