ci: remove PHPUnit and add yoast polyfills as dev dependency

.github/workflows/.pipeline.yml

@@ -72,7 +72,6 @@ jobs:
         run: |
           export PATH="$HOME/.composer/vendor/bin:$PATH"
           composer install --no-interaction
-          composer global require "phpunit/phpunit:7.5.20"
 
       - name: Run Lint
         run: npm run lint

composer.json

@@ -20,19 +20,20 @@
 	"config": {
 		"platform": {
 			"php": "7.3"
-		}
+		},
+    "sort-packages": true
 	},
 	"require": {
-		"php": ">=7.1",
+		"php": ">=7.3",
 		"composer/installers": "^1.7",
 		"pressbooks/mix": "^2.1",
 		"soberwp/intervention": "1.2.0-p",
 		"spatie/color": "^1.1"
 	},
 	"require-dev": {
-		"phpunit/phpunit": "^7",
+		"pressbooks/coding-standards": "^1.1",
 		"wpreadme2markdown/wp2md": "^3.0",
-		"pressbooks/coding-standards": "^1.1"
+		"yoast/phpunit-polyfills": "^1.0"
 	},
 	"scripts": {
 		"test": [