Allow SortDirection in EntityRepository::findBy() and findOneBy()

[GromNaN]

EntityRepository::findBy() and findOneBy() now accept and document \SortDirection values for $orderBy, while 'asc'/'desc' strings keep working without a deprecation.

The ObjectRepository interface of doctrine/persistence 3.x and 4.x documents strings as the only valid order values, so the string form stays accepted on the repository API. As an implementer of the interface, EntityRepository can widen its documented parameter type, so passing \SortDirection::Ascending or \SortDirection::Descending is now allowed and documented there, matching what QueryBuilder, Expr\OrderBy and the mapping API already accept. Native signatures are unchanged.

Breaking change

This is a static-analysis BC break for custom repositories that override findBy() or findOneBy() and forward $orderBy to parent::*(): the documented $orderBy type now also accepts \SortDirection, so an override whose docblock reproduces the previous type (the ObjectRepository string union for findBy(), or array<string, string> for findOneBy()) is reported as incompatible by static analysis. Widen the override docblock accordingly. ObjectRepository itself is unchanged.

Static analysis only allows SortDirection when the repository is typed as EntityRepository (or a subclass), not as Doctrine\Persistence\ObjectRepository, until doctrine/persistence 5 is required. The deprecation scope is clarified in UPGRADE.md accordingly.

Changes:

• widen the $orderBy phpdoc of EntityRepository::findBy() and findOneBy(), and of EntityPersister::load()
• add functional tests and a static-analysis fixture
• update UPGRADE.md and the reference docs

[greg0ire]

Since this contains a breaking change, maybe we should mute the issue on 3.7.x, and this PR should target 3.8.x? On the other hand, it's just phpdoc/static analysis. I'm on the fence…