Drop support for PHP 7.3.

Author: prwater

composer.json

@@ -8,7 +8,7 @@
   ],
   "license": "MIT",
   "require": {
-    "php": ">=7.3",
+    "php": ">=7.4",
     "ext-mbstring": "*",
     "plaisio/error-logger": "^1.2.0",
     "plaisio/helper-html": "^2.0.0",