Mark MtRandGenerator and OpenSslGenerator as deprecated

src/Generator/MtRandGenerator.php

@@ -18,6 +18,10 @@ namespace Ramsey\Uuid\Generator;
  * MtRandRandomGenerator provides functionality to generate strings of random
  * binary data using the `mt_rand()` PHP function
  *
+ * @deprecated 4.0.0 The mt_rand() function is not a reliable source of
+ *     randomness. The default RandomBytesGenerator, which uses the
+ *     random_bytes() function, is recommended as the safest and most reliable
+ *     source of randomness.
  * @link http://php.net/mt_rand
  */
 class MtRandGenerator implements RandomGeneratorInterface