Do not specify PHP version in root composer

The intention of the root composer file is just to ensure that Whippet
is available for use in local development (or in automated workflows)
without needing to be globally installed.

Any PHP version specified here neither indicates what PHP version the
app is intended to run on (as none of the dependencies here are used in
deployed code), or should affect the output of any Whippet commands run.

So, for maximum compatibility as we're in the midst of the PHP7-to-8
switchover, let's not indiciate a PHP version here at all.

composer.json

@@ -1,9 +1,4 @@
 {
-    "config": {
-        "platform": {
-            "php": "7.4.3"
-        }
-    },
     "require-dev": {
         "dxw/whippet": "^2.0"
     }