Update PHP version requirement in readme

[jmejmejme]

Describe the bug

In the readme file and on the wordpress.org plugin page, "Requires PHP: 5.6"

In composer.php:

"config": {
		"platform": {
			"php": "7.4"

Installing the plugin in any PHP version lower than 7.2 results in a critical error printing a message from vendor/composer/platform_check.php

Site information

Popup Maker version: 1.20.2

WordPress version: 6.2

PHP version: <7.2.0

Expected behavior

PHP version requirement in the readme should reflect plugin requirements

Current behavior

PHP version requirement does not reflect actual plugin requirements

Steps to reproduce

1. Install plugin in a WordPress site running on a PHP version lower than 7.2.0

Additional context

Requiring PHP 7.2 or 7.4 is totally reasonable! Just a nudge to update the readme.