Increase minimum php version to 7.3

Author: joelambert

composer.json

@@ -1,7 +1,7 @@
 {
     "name": "rareloop/primer-core",
     "require": {
-        "php": ">=7.1.3",
+        "php": ">=7.3",
         "symfony/finder": "^4.2.5|^5.0",
         "twig/twig": "^2.6",
         "tightenco/collect": "^5.6.0",