Support for PHP 7.3 dropped

[jbtronics]

The support for PHP 7.3 is dropped since commit 751eff, so Part-DB now required PHP 7.4 at least.

This is mostly because newer doctrine versions requires PHP 7.4 and these versions offers much better SQLite support (as they finally support foreign keys on SQLite properly).

PHP 7.3 is End-Of-Life since December 2021 and all the currently supported mainstream distributions ship at least PHP 7.4, so the impact should not be big.