chore: bump phpunit/phpunit to ^12.5.22 (#4585)

Unblocks composer install on flarum/testing consumers: the PHPUnit 11.x
line has no fix for GHSA-qrr6-mg7r-m243 (PKSA-5jz8-6tcw-pbk4), so every
11.x version is filtered by Composer's advisory check.

PHPUnit 12 requires PHP 8.3+, which matches the 2.x floor. Flarum's own
test suites already use PHP 8 attributes exclusively; third-party
extensions still using @test / @dataProvider docblock annotations will
need to migrate.

Closes #4584

Author: imorland

composer.json

@@ -180,7 +180,7 @@
         "larastan/larastan": "^3.8",
         "mockery/mockery": "^1.5",
         "phpstan/phpstan": "^2.1",
-        "phpunit/phpunit": "^11.0",
+        "phpunit/phpunit": "^12.5.22",
         "symfony/var-dumper": "^7.0"
     },
     "config": {

php-packages/testing/composer.json

@@ -10,7 +10,7 @@
     "require": {
         "ext-json": "*",
         "mockery/mockery": "^1.5",
-        "phpunit/phpunit": "^11.0"
+        "phpunit/phpunit": "^12.5.22"
     },
     "require-dev": {
         "flarum/core": "*@dev",