docs: add/fix PHPDoc types in MockCache

[kenjis]

Description
PHPStan update now causes an error.
PHPDoc types have been fixed along with it.

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  Line   system/Test/Mock/MockCache.php                                                                      
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         Ignored error pattern #^Cannot unset offset string on array<int, int>\|null\.$# in path             
         /Users/kenji/work/codeigniter/official/CodeIgniter4/system/Test/Mock/MockCache.php was not matched  
         in reported errors.                                                                                 
 ------ ---------------------------------------------------------------------------------------------------- 

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