Raise minimal supported PHP version to 7.4

PHP 7.3:

- Active support ended on 6th December 2020.
- Security support ends on 6th December 2021.

See currently supported versions: https://www.php.net/supported-versions.php

remp/crm#2068

composer.json

@@ -5,7 +5,7 @@
   "homepage": "https://remp2020.com",
   "license": "MIT",
   "require": {
-    "php": "^7.3",
+    "php": "^7.4",
     "mpdf/mpdf": "^8.0",
     "jkuchar/pdfresponse": "^0.6"
   },