For xcoms pushed from different tasks, having the same execution_date and in the same Dag, the result of the xcom_pull with key set as None is not deterministic in its order

**Apache Airflow version**: 2.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
When tasks in the same dag push Xcoms, the result of the xcom_pull comes in any order. The order is not deterministic.

**What you expected to happen**:

I expected the order to be deterministic.

**How to reproduce it**:

Run the provided example_xcom dag in the repository multiple times


tests/models/test_taskinstance.py
dag = DAG('dag', start_date=DEFAULT_DATE)
        with dag:
            op1 = DummyOperator(task_id='test_op_1', owner='test')
            op2 = DummyOperator(task_id='test_op_2', owner='test')
            op3 = DummyOperator(task_id='test_op_3', owner='test')
            op1 >> op2 << op3
        
        self.assertIn(op2, op1.downstream_list)
        self.assertIn(op2, op3.downstream_list)
    @patch.object(DAG, 'get_concurrency_reached')
    def test_requeue_over_dag_concurrency(self, mock_concurrency_reached):
        mock_concurrency_reached.return_value = True
        dag = DAG(
            dag_id='test_requeue_over_dag_concurrency',
            start_date=DEFAULT_DATE,
            max_active_runs=1,
            concurrency=2,
        )
        task = DummyOperator(task_id='test_requeue_over_dag_concurrency_op', dag=dag)
        ti = TI(task=task, execution_date=timezone.utcnow(), state=State.QUEUED)
        
        with create_session() as session:
            session.add(ti)
            session.commit()
        ti.run()
        self.assertEqual(ti.state, State.NONE)
    def test_requeue_over_task_concurrency(self):