Improve Code Quality by adding as much as possible typed properties

[llaville]

Hello,

As this package requires at least PHP 7.4, I think we could generalized, as much as possible, Typed properties implementation.

This is the main goal of this PR. Second objective : migrate PHPUnit config file to schema 10.5 and keep a phpunit.xml.dist.legacy for previous version v9. Third, fix the PHP constraint on composer.json even if old syntax is still supported (but for how long time).

Current test results of version 1.6.1

PHPUnit 10.5.8 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.15
Configuration: /shared/backups/forks/async/phpunit.xml.dist

....................................                              36 / 36 (100%)

Time: 00:14.361, Memory: 28.76 MB

There was 1 PHPUnit test runner deprecation:

1) Your XML configuration validates against a deprecated schema. Migrate your XML configuration using "--migrate-configuration"!

OK, but there were issues!
Tests: 36, Assertions: 69, Deprecations: 1.

After PHPUnit config file migration and added typed properties

PHPUnit 10.5.8 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.15
Configuration: /shared/backups/forks/async/phpunit.xml.dist

....................................                              36 / 36 (100%)

Time: 00:14.315, Memory: 28.76 MB

OK (36 tests, 69 assertions)