refactor: add precise return type to CodeIgniter::getPerformanceStats()

[soccerlover29]

Description

Adds a specific return-type annotation to CodeIgniter::getPerformanceStats() so static analysis knows exactly what's in the returned array (startTime: float|null and totalTime: float), instead of just "array". Removes the now-unneeded PHPStan baseline suppression for this method.

Contributes to #8732 (PHPStan "no value type specified in iterable type array" cleanup).

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[mergeable]

Hi there, soccerlover29! 👋

Thank you for sending this PR!

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Sincerely, the mergeable bot 🤖

[paulbalandan]

Thank you, @soccerlover29