[armanist]

Closes #208. Follows quantum-php/framework#595.

What changed

• composer.json requires php ^8.1.
• The CI matrix is PHP 8.1, 8.2 and 8.3.
• PHPStan phpVersion is 80100 and Rector uses UP_TO_PHP_81.
• The Rector 8.1 level set is applied to the project code (second commit): 6 properties in the DTOs and services become readonly, and 5 null values are cast to strings before trim(), strtotime() and preg_replace(), which avoids the PHP 8.1 null deprecation.
• The README states PHP 8.1 as the minimum.
• CHANGELOG.md has a breaking entry and a changed entry under 3.1.0.

Why

quantum/framework requires PHP 8.1 since 3.1, so CI on PHP 8.0 can no longer install it, and the skeleton's own constraint was wrong.

Breaking changes

• Projects on PHP 8.0 cannot install this version.

Testing

• Locally on PHP 8.1: the Unit suite (19 tests, 72 assertions) and the Feature suite (27 tests, 147 assertions) pass before and after the Rector changes, and cs:check, PHPStan and rector:check pass.
• CI runs the tests and a short end-to-end check (it starts the app and requests /api/posts) on PHP 8.1, 8.2 and 8.3, and all pass.