dev: update PHPStan

[Soare-Robert-Daniel]

All Submissions:

• Have you followed the Optimole Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

• Update PHPStan to V2
• Fix the new reported errors for Level 5
• Add level 6 with baseline

Closes # .

How to test the changes in this Pull Request:

1. The plugin should work normally

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?