[8.x] Replace direct application class mention by illuminate contract

[alexmakii]

This PR replaces Illuminate\Foundation\Application by contract Illuminate\Contracts\Foundation\Application.

Illuminate\Foundation\Application was accidentally added by #35204.

Reasons:
As illuminate/database can be used outside of the Laravel framework (with Lumen in my case) - direct mention of Laravel only classes should be avoided when it's possible.

[alexmakii]

@taylorotwell Can you you explain why this PR was closed ?

[BrandonSurowiec]

@alexmakii One reason would be that the tests didn't pass:

There were 2 errors:

1) Illuminate\Tests\Database\DatabaseEloquentFactoryTest::test_resolve_nested_model_factories
Use of undefined constant Container - assumed 'Container' (this will throw an Error in a future version of PHP)

/home/runner/work/framework/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:713
/home/runner/work/framework/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:722
/home/runner/work/framework/framework/tests/Database/DatabaseEloquentFactoryTest.php:424

2) Illuminate\Tests\Database\DatabaseEloquentFactoryTest::test_resolve_non_app_nested_model_factories
Use of undefined constant Container - assumed 'Container' (this will throw an Error in a future version of PHP)

/home/runner/work/framework/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:713
/home/runner/work/framework/framework/src/Illuminate/Database/Eloquent/Factories/Factory.php:722
/home/runner/work/framework/framework/tests/Database/DatabaseEloquentFactoryTest.php:443

[taylorotwell]

@alexmakii Literally had code that was invalid PHP in it - meaning you didn't test it even by running it.