Expand version constraints for 7.2 and 7.3 support

The most recent version of Infection PHP that will run on 7.2 is 0.15.x.
v0.19 adds support for php 8, but also drops support for versions older
than 7.4 meaning we also need to allow for 0.18.x to get 7.3 working.

Author: dshoreman

composer.json

@@ -7,11 +7,11 @@
         "process-timeout": 0
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.4",
+        "phpunit/phpunit": "^8.5 || ^9.4",
         "vimeo/psalm": "^4.1",
-        "infection/infection": "^0.19.2",
+        "infection/infection": "^0.15.3 || ^0.18.2 || ^0.19.2",
         "spatie/phpunit-watcher": "^1.22",
-        "phpunit/php-invoker": "^3.1",
+        "phpunit/php-invoker": "^2.0 || ^3.1",
         "pluswerk/grumphp-config": "^4.0"
     },
     "license": "MIT",