composer.json - Raise PHP minimum to 7.3

For core, it's been 7.3 for a while... and if we're doing cv v0.4 for minor-breaking-thing, then
this is another good minor-breaking-thing.

Author: totten

composer.json

@@ -9,7 +9,7 @@
         }
     ],
     "require": {
-        "php": ">=7.2.0",
+        "php": ">=7.3.0",
         "lesser-evil/shell-verbosity-is-evil": "~1.0",
         "symfony/console": "^4",
         "symfony/process": "^4",
@@ -26,7 +26,7 @@
     ],
     "config": {
         "platform": {
-            "php": "7.2.0"
+            "php": "7.3.0"
         },
         "bin-dir": "bin",
         "allow-plugins": {