Issue #3619679: Fix the two PHPStan errors in RestMenuItemsResource

new.static: the class is deliberately extensible - getMenuItems(), getElementValue() and setup() are protected - so making it final is not an option. Adding @phpstan-consistent-constructor states that the constructor signature is meant to stay the same in subclasses, which is what PHPStan needs to know.

drupal.entityStoragePropertyAssignment: $menuLinkContentStorage is now deprecated and checkContentFields() loads the storage from the entity type manager at the call site. The property is still assigned in the constructor so subclasses reading it keep working; it is removed in 4.0.0. That assignment is the backwards compatibility layer itself, so both that rule and property.deprecated are silenced on the one line, with a comment explaining why.

Closes #3619679