Using deprecated function

[Tung Du]

Describe the bug

mcrypt_create_iv was DEPRECATED in PHP 7.1.0, and REMOVED in PHP 7.2.0. Our plugin is using mcrypt_create_iv so it can't be used for PHP ≥ 7.2

Expected behavior
WP Hammer works normally with PHP ≥ 7.2

Environment information

• Other installed plugin(s) and version(s): WP Local Docker with PHP 7.4

[Tung Du]

As PHP docs suggested, we can use random_bytes as a replacement for mcrypt_create_iv.