Allow classname in 'value' attribute of xml discriminator-mapping field

[MatteoFeltrin]

fixes #11449

In relation to #10630
This PR allows a more loosen validation of the value attribute of discriminator-mapping field, using orm:type in the XSD schema as it was done in the linked pull request.
The test I wrote loads the entity enabling the validation against XSD schema, thus expects no exception to be thrown.

[Ocramius]

Let's declare string here, or directly use constructor property promotion

[MatteoFeltrin]

PHPStorm gives these errors:

Typed properties are only allowed since PHP 7.4

Constructor property promotion is only allowed since PHP 8.0

Composer.json

"require": {
        "php": "^7.1 || ^8.0",
        ...,
}