Update dependencies: PHPUnit, etc. (#260)

PHP 5.4 → 7.4 minimum
PHPUnit 8 → 9.6
Yoast PHPUnit Polyfills 1 → 4
Composer Installers 1 → 2
Symfony YAML 5.4.45 → 5.4.53

Author: sirreal

.gitignore

@@ -1,3 +1,4 @@
 vendor
 coverage
 node_modules
+.phpunit.result.cache

README.md

@@ -7,7 +7,7 @@ We are currently looking for contributors to help us complete the work on the pa
 There is a guide to developing for developer.wordpress.org in the [WordPress documentation handbook](https://make.wordpress.org/docs/handbook/projects/devhub/)
 
 ## Requirements
-* PHP 5.4+
+* PHP 7.4+
 * [Composer](https://getcomposer.org/)
 * [WP CLI](https://wp-cli.org/)
 

composer.json

@@ -33,7 +33,7 @@
 	],
 	"require"    : {
 		"php"                      : ">=7.4",
-		"composer/installers"      : "~1.0",
+		"composer/installers"      : "^2.0",
 		"phpdocumentor/reflection"          : "^3.0@dev",
 		"phpdocumentor/reflection-docblock" : "dev-release/2.x as 2.0.5",
 		"erusev/parsedown"         : "~1.7",
@@ -42,9 +42,9 @@
 		"psr/log"                  : "~1.0"
 	},
 	"require-dev" : {
-		"phpunit/phpunit": "~8",
+		"phpunit/phpunit": "^9.6",
 		"spatie/phpunit-watcher": "^1.23",
-		"yoast/phpunit-polyfills": "^1.0"
+		"yoast/phpunit-polyfills": "^4.0"
 	},
 	"scripts"    : {
 		"test": "phpunit",