test: fix PHPDoc types in CastBase64

[kenjis]

Description

1 file with changes
===================

1) tests/_support/Entity/Cast/CastBase64.php:21

    ---------- begin diff ----------
@@ @@
      *
      * @param mixed $value  Data
      * @param array $params Additional param
-     *
-     * @return mixed
      */
     public static function get($value, array $params = []): string
     {
@@ @@
      *
      * @param mixed $value  Data
      * @param array $params Additional param
-     *
-     * @return mixed
      */
     public static function set($value, array $params = []): string
     {
    ----------- end diff -----------

Applied rules:
 * RemoveUselessReturnTagRector

https://github.com/codeigniter4/CodeIgniter4/actions/runs/7324536612/job/19948207442?pr=8364

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

@ddevsr Thank you for the review!