Prepare Quantum Project for PHP 8 baseline alignment

[armanist]

Summary

Align the Quantum Project repository with the framework's PHP 8 baseline work so the starter project no longer advertises PHP 7.4 support.

Goals

• Raise the project composer.json PHP requirement from ^7.4 || ^8.0 to ^8.0
• Update CI to test PHP 8.0, 8.1, and 8.2
• Raise PHPStan from PHP 7.4 / level 5 to a PHP 8 baseline with an appropriate level review
• Keep Rector aligned with the PHP 8 baseline and apply only bounded modernization changes
• Review OpenAPI-related project code and docs for any baseline-sensitive updates, while retaining Doctrine annotations if the current swagger-php path still requires PHP 8.1+
• Update changelog / support docs to reflect the new PHP support policy

Notes

• This should stay separate from the framework issue so project-specific changes can be reviewed and merged independently.
• If OpenAPI attributes are not compatible with the project's current PHP baseline/tooling path, keep the existing annotations and document the reason clearly.

Acceptance Criteria

• Project package metadata requires PHP ^8.0
• CI matrix reflects the PHP 8.0, 8.1, 8.2 support policy
• Static analysis and Rector configuration are updated and passing
• Relevant docs reflect the project's PHP 8 support baseline
• Validation commands pass locally