[1.x] Improve PHP 8.4+ support by avoiding implicitly nullable types

Builds on top of #172 and ports #181 to v1.

Run tests on PHP 8.4 and update test environment

In PHP8.1 ReflectionProperty::setAccessible was made a no-op through https://wiki.php.net/rfc/make-reflection-setaccessible-no-op in PHP8.5 it is now throwing a deprecation warning by: https://wiki.php.net/rfc/deprecations_php_8_5#deprecate_reflectionsetaccessible

While I couldn't pin it to the exact commit/PR/NEWS/UPGRADING item, PHP8.5 has seen a number of improvements to streams to make them less buggy. As a result, this test, which already confirmed the expected behavior is now failing because an error is tossed.

This changes ensures we can continue run PHPUnit on PHP 7.2 by updating PHPUnit to 8.5. This is due to recent improvements in composer as discussed in reactphp/event-loop#284.

…environment-for-PHP-7.2

[1.x] Update test environment for PHP 7.2 to compatible PHPUnit version

[1.x] Improve PHP 8.5+ support by avoiding deprecated method calls in tests