docs: add API Platform 4.4 to 5.0 upgrade guide

[soyuka]

Summary

• Adds the missing ## API Platform 4.4 to 5.0 section to core/upgrade-guide.md (was entirely absent — 0/10 breaking-change coverage per the docs audit).
• Every claim verified against api-platform/core at upstream/main (the shipped PHP source, not just CHANGELOG prose) and against CHANGELOG.md's v5.0.0-beta.1 entry.

What's covered (Breaking Changes)

• JSON:API use_iri_as_id now defaults to false — before/after payload, Symfony + Laravel opt-out, cross-link to jsonapi.md.
• DeserializeProvider constructor: TranslatorInterface 4th arg dropped, DenormalizationViolationFactoryInterface moves 5th → 4th; api-platform/state no longer requires symfony/translation-contracts.
• Request::getContentType() fallbacks dropped (core#8517) — internal, tied to the ^7.4 || ^8.0 floor.
• The full core#8367 removed-API batch: config keys (query_parameter_validation, enable_link_security, resource_class_directories, graphql_playground, varnish_urls, xkey), ObjectMapperProcessor split into Input/Output processors, DefinitionNameFactory's $distinctFormats arg, ValidationException's string-message ctor, ApiTestCase::$alwaysBootKernel default, automatic short-name deduplication, and the now-explicit api_assign_object_to_populate context flag.
• Legacy PropertyInfo Type system removed for symfony/type-info (core#8364): ApiProperty::$builtinTypes/getBuiltinTypes()/withBuiltinTypes() → nativeType; GraphQL TypeConverterInterface::getType() → convertPhpType(); ContextAwareTypeBuilderInterface::isCollection() removed.
• PropertyAwareFilterInterface::getProperties() now a real interface method; JSON:API error status is now a string (core#8366).
• JSON-LD /contexts/Error and /contexts/ConstraintViolationList no longer special-cased; SerializerContextBuilder no longer injects uri_variables (core#8402).
• UniqueConstraintViolationException → 422 by default (core#8478).
• Doctrine filters: ComparisonFilter gains [between], RangeFilter deprecated (removed in 6.0), DateFilter/ExistsFilter become standalone — kept short, cross-linked to doctrine-filters.md (owned by another in-flight PR).
• CompatibleValueResolverInterface removed; PayloadArgumentResolver implements ValueResolverInterface directly.

Deprecations

• FilterInterface::getDescription() removal explicitly stated as deferred to 6.0 (core#8513) — it is NOT removed in 5.0, along with #[ApiFilter]/Operation::$filters/AbstractFilter.

Notes

• One-line pointer that ApiTestCase moved to api-platform/test (full treatment left to feat: extract ApiTestCase in its own api-platform/test package #2276, already in flight).

Corrections vs. the source task list

• "automatic short-name deduplication" is not a removed API per se — verified in MetadataCollectionFactoryTrait::deduplicateShortNames() (core commit e22e74464) that the deduplicate_resource_short_names opt-in flag and its deprecation path are removed, making deduplication unconditional. Phrased it that way rather than as an outright removal.
• Config key paths verified precisely from Configuration.php: validator.query_parameter_validation, graphql.graphql_playground, http_cache.invalidation.varnish_urls, http_cache.invalidation.xkey (nested), vs. root-level enable_link_security and resource_class_directories.
• assign_object_to_populate: the 5.0 change removes DeserializeProvider's own HTTP-method-based fallback (verified in commit e22e74464's diff of DeserializeProvider.php); the flag is instead set upstream by MainController/DeserializeListener (Symfony) and ApiPlatformController (Laravel), confirmed by commit 58f28b8e7.

Left out / not verified

• Did not touch core/jsonapi.md:11 ("This will become the default in 5.x") even though it's now stale/wrong per this change (W6 in the audit) — out of scope for this PR (edits limited to core/upgrade-guide.md).
• Did not restate doctrine-filters.md filter migration content — another PR owns that page.

Test plan

• npx prettier@3.9.5 --check core/upgrade-guide.md --prose-wrap always passes.
• Whole-repo npx prettier@3.9.5 --check "**/*.md" --prose-wrap always shows only 2 pre-existing failures (core/doctrine-filters.md, core/filters.md), confirmed present on upstream/main before this change (unrelated, out of scope).