docs: replace type mixed in Cookie class.

[ping-yee]

Description
See #6310

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]

Error: Parameter #2 $array of function implode expects array<string>, array<int, bool|int|object|string> given.
 ------ ---------------------------------------------------------------- 
  Line   system/Cookie/CookieStore.php                                   
 ------ ---------------------------------------------------------------- 
  146    Parameter #2 $array of function implode expects array<string>,  
         array<int, bool|int|object|string> given.                       
 ------ ---------------------------------------------------------------- 

https://github.com/codeigniter4/CodeIgniter4/actions/runs/4407171630/jobs/7720467402

[ping-yee]

@kenjis I have been fixed this problem of phpstan.

[kenjis]

Thank you.