Bug: Generated proxy method getId() casts to int and breaks when used with enums

**The bug**

The generated proxy class for an entity has the following code for the getId() method:
```
/**
* {@inheritDoc}
*/
public function getId(): ?ExampleEnum
{
if ($this->__isInitialized__ === false) {
return (int) parent::getId();
}

$this->__initializer__ && $this->__initializer__->__invoke($this, 'getId', []);

return parent::getId();
}
```
The problem is the line with `return (int) parent::getId();` because it's casting the return value to `int`, even though the method is expected to return `?ExampleEnum`.

**Current behavior**

An error is thrown:

`Proxies\\__CG__\\App\\Entity\\ExampleEntity::getId(): Return value must be of type ?ExampleEnum, int returned`

**How to reproduce**

To reproduce this, you need:

1. an entity with a PHP enum class as the primary key
2. a second entity that has a ManyToOne relationship with the first
3. load an instance of the second entity, and then try to fetch the id of the related entity (the one with the enum PK)
4. since the related entity is not yet loaded, the proxy method will be used
5. error occurs

```
enum ExampleEnum: int
{
Case A = 1;
Case B = 2;
}

class ExampleEntity
{
#[ORM\Column(name: 'id', type: Types::INTEGER, nullable: false, enumType: ExampleEnum::class)]
#[ORM\Id]
#[ORM\GeneratedValue(strategy: 'IDENTITY')]
private ?ExampleEnum $id = null;

public function getId(): ?ExampleEnum
{
return $this->id;
}
}

class AnotherEntity
{
#[ORM\Column(name: 'id', type: Types::INTEGER, nullable: false)]
#[ORM\Id]
#[ORM\GeneratedValue(strategy: 'IDENTITY')]
private ?int $id = null;

public function getId(): ?int
{
return $this->id;
}

#[ORM\ManyToOne(targetEntity: ExampleEntity::class)]
#[ORM\JoinColumn(name: 'entityId', referencedColumnName: 'id', nullable: false)]
private ExampleEntity $relatedEntity;

public function getRelatedEntity: ExampleEntity
{
return $this->relatedEntity;
}
}

$foo = $em->getRepository(AnotherEntity::class)->find(1);

$enum = $foo->getRelatedEntity()->getId(); // fails here
```

**Expected behavior**

The getId() method should return the correct enum class instead of an int.

**Workaround**

Avoid using the proxy by using eager loading, or fetching some other property to initialize the object.

**Potential solution**

I can see the offending code is here: https://github.com/doctrine/common/blob/0a357f1bb08a95a2b170f7db19c9e775420b40b3/src/Proxy/ProxyGenerator.php#L936-L950

You could simply remove the `$cast` variable, it's not necessary in my opinion, since you already declare the return type thanks to:

`$this->getMethodReturnType($method)`

Thanks!