test: drop stale serializer 8.1 getErrors() deprecation expectation

[soyuka]

Problem

main CI fails on PHP 8.4/8.5 (all DB + Symfony variants) with 2 test failures:

• EnumDenormalizationValidationTest::testInvalidBackedEnumValueWithCollectDenormalizationErrors
• NullOnNonNullablePropertyTest::testNullOnNonNullablePropertyReturns422WhenCollectingErrors

Both:

Expected deprecation with message "Since symfony/serializer 8.1: The
"...PartialDenormalizationException::getErrors()" method is deprecated,
use "...getNotNormalizableValueErrors()" instead." was not triggered

Cause

Since #8211 the denormalization violation factories
(src/Validator/DenormalizationViolationFactory.php,
src/Laravel/State/DenormalizationViolationFactory.php) guard the call:

$errors = method_exists($exception, 'getNotNormalizableValueErrors')
    ? $exception->getNotNormalizableValueErrors()
    : $exception->getErrors();

On symfony/serializer >=8.1 the new method exists, so the deprecated
getErrors() is never called and the deprecation never fires. The
expectUserDeprecationMessage assertions are stale — production code was
fixed to avoid the deprecation, but the expectations weren't removed.

Fix

Remove the expectUserDeprecationMessage blocks from both tests plus the
now-unused VersionParser / InstalledVersions / IgnoreDeprecations
imports.

Verified against symfony/serializer:^8.1 (4/4 pass) and unchanged on
8.0 (the block was already skipped there). php-cs-fixer clean.

Refs #8211