PHP 7.3 support

[eclipxe13]

This library (as of 2023-02-27) is still compatible with PHP 7.3.
Soon the support will be drop according to PHP Supported Versions.
The test matrix cannot run using PHP 7.3 since Sunshine PHP (implementation of PSR libraries) support PHP 7.4 or higher.

Close this issue when:

1. PHP library support is updated to PHP 7.4 or higher.
2. Someone provide a patch to test using PHP 7.3.

(Even with the option 2, the support for PHP 8.1 or lower will be removed soon)

[eclipxe13]

Related PR: #13