[BUG] Order of `@Model` and `@DiscriminatorValue` decorators can result in wrong discriminator value
## Information

- **Version:** 7.13.1
- Packages: `@tsed/schema`, `@tsed@mongoose`

Order of `@Model` and `@DiscriminatorValue` decorators can result in wrong (default) discriminator value.

## Example

```ts
import {DiscriminatorKey, DiscriminatorValue, Property, Required} from "@tsed/schema";

@Model()
export class Event {
  @DiscriminatorKey()
  type: string;

  @Property()
  value: string;
}

@Model()
@DiscriminatorValue("page_view")
export class PageView extends Event {
  @Required()
  url: string;
}
```

Because of this line here:

https://github.com/tsedio/tsed/blob/production/packages/orm/mongoose/src/decorators/model.ts#L51

the custom discriminator value gets overwritten by the class name, based on the order of the decorators.

This works:

```ts
@DiscriminatorValue("page_view")
@Model()
export class PageView extends Event {
// ...
```

Also still works (but deprecated):
```ts
@Model({ discriminatorValue: "page_view" })
export class PageView extends Event {
// ...
```

## Acceptance criteria

- [ ] Discriminator value should not depend on the order of the `@Model` and `@DiscriminatorValue` decorators.

