build: Pin PHPUnit to 9.5.28

Change-Id: I62672d2d00b0efb14291ba59a0ff09a7857ed7f7

Author: jdforrester

composer.json

@@ -25,7 +25,7 @@
 	"require-dev": {
 		"mediawiki/mediawiki-phan-config": "0.12.0",
 		"php-parallel-lint/php-parallel-lint": "^1.3.0",
-		"phpunit/phpunit": "^9.5",
+		"phpunit/phpunit": "9.5.28",
 		"squizlabs/php_codesniffer": "^3.6.0"
 	},
 	"scripts": {