Remove hardcoded version and add PHP requirement to composer.json

Author: mrcasual

composer.json

@@ -1,9 +1,11 @@
 {
   "name": "gravitykit/gravity-forms-zero-spam",
   "description": "Enhance Gravity Forms to include effective anti-spam measures.",
-  "version": "1.6.0",
   "type": "wordpress-plugin",
   "license": "GPL-2.0-or-later",
+  "require": {
+    "php": ">=7.4"
+  },
   "require-dev": {
     "squizlabs/php_codesniffer": "*",
     "wp-coding-standards/wpcs": "^3.0",