chore(phpstan): upgrade phpstan to 2.x

[jacquesbh]

What

• Bump dev tooling: phpstan/phpstan ^1.8.4 → ^2.2, phpstan/phpstan-doctrine ^1.3.2 → ^2.0, phpstan/phpstan-webmozart-assert ^1.1 → ^2.0 (installed: 2.2.16 / 2.0.28 / 2.0.0)
• phpstan.neon: remove the now-unmatched missingType.generics ignore (keep missingType.iterableValue)
• Quote @SuppressWarnings("PHPMD.*") annotations across 9 files: PHPStan 2 strictly parses PHPDoc and flags the unquoted form; PHPMD behaviour verified unchanged (quoted form honoured)
• Add @return class-string<\Symfony\Component\Form\FormTypeInterface> docblocks on the conditional form-type getters (ImageType::getImageType(), VideoType::getImageType() / getVideoType()) so FormBuilderInterface::add() accepts them
• WysiwygType: drop a redundant ?? self::DEFAULT_LOCALE (explode()[0] is never null) — behaviour-identical
• Run the fixer on 2 pre-existing phpcs-flagged files (src/UiElement/Metadata.php, src/Form/Type/UiElement/SeparatorType.php: closures → static closures)

No runtime behavior changes.

Verification

• composer validate --strict, phpmd, phpcs, phpspec (2 specs / 14 examples), yaml, twig, schema and container lints: all green
• phpstan (level 8, PHPStan 2.2.16): 7 remaining errors, all in src/Uploader/FixtureFileUploader.php (Gaufrette class.notFound ×6 + alreadyNarrowedType ×1). These are exactly what Replace Gaufrette by FlySystem #286 fixes; this PR intentionally does not touch that file to avoid overlap. Rebase/merge after Replace Gaufrette by FlySystem #286.

Notes

• Fresh dependency resolution installs sylius/sylius 2.3.0, which no longer ships Gaufrette — hence the errors above exist on 3.x today, independent of this upgrade.

Update: rebased onto #286's head so this PR runs on top of the Flysystem migration and CI can be green. Until #286 merges, this PR's diff also contains #286's three files (config.yaml, services.yaml, FixtureFileUploader.php). Once #286 merges, this branch will be rebased onto the new 3.x tip and the diff will show only the PHPStan upgrade.

CI note: the php (8.2) job failed at make test.schema due to doctrine/orm 3.7 (fresh resolution — sylius-standard v2.1.4 has no lock file) tripping on Sylius's unused ShipmentUnit mapping (Sylius/Sylius#19235). The Makefile now pins doctrine/orm ~3.5.0 in the test application, matching what the 8.3 job already installs from sylius-standard v2.1.6's lock. Unrelated to the PHPStan upgrade itself (php 8.3 passes the full suite).