Update to PHP 8.2

[ebitkov]

PHP 8.1 is not actively supported anymore. This pull requests aims to update the project to PHP 8.2.

• added PHP 8.2 as a requirement to the composer.json (best practice)
• added a note to the README to be more precise about the required PHP version
• added type hints to the core, controllers and models (best practice)
• updated bootstrap.sh (@panique could you review that? I'm not using Vagrant)

Some questions I had while updating

• should we add ext-pdo to the composer.json, since we are providing PDO models?
• should we switch to const ... instead of define(...) in the application/config/config.php and public/index.php?