[8.x] Fix database migrations $connection property

[derekmd]

Fixes #41153 #40097 #38777 #36596 #31060

The docs show the below migration will be run on the named database connection:

return new class extends Migration
{
    protected $connection = 'pgsql';

    function up()
    {
        Schema::create('users', function (Blueprint $table) {
            // ...
        });
    }
}

However Schema facade-built queries will run on the app's default database connection or the --database command line option's named connection. The migration also requires Schema::connection('pgsql'):

return new class extends Migration
{
    protected $connection = 'pgsql';

    function up()
    {
        Schema::connection('pgsql')->create('users', function (Blueprint $table) {
            // ...
        });
    }
}

This fix allows the first example to work as documented for the up() & down() methods. The selected connection priority is:

1. Schema::connection()
2. migration class property $connection
3. php artisan migrate* command line option --database
4. config('database.default')

I also added tests for the Migrator@usingConnection() method added in 7.x for the migration commands.

[rawmarius]

Thank you so much for the fix! It would be nice if the priority would be added to the documentation.
Regards!