refactor: update rector, psalm and add strict types

[michalsn]

Description
This PR enables declare(strict_types=1) and void return types across all source and test files. It updates Rector from PHP 7.4 to PHP 8.2 as the target, bumps the Psalm PHP version from 8.1 to 8.2, and raises the minimum framework requirement from ^4.2.3 to ^4.3. It also adds readonly to private properties where applicable.

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

[michalsn]

Thank you @paulbalandan