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


airflow/models/taskinstance.py
"""
        Returns whether a task is in UP_FOR_RETRY state and its retry interval
        has elapsed.
        """
        
        return self.state == State.UP_FOR_RETRY and not self.ready_for_retry()
    @provide_session
    def are_dependents_done(self, session=None):
        """
        Checks whether the immediate dependents of this task instance have succeeded or have been skipped.
        This is meant to be used by wait_for_downstream.
        This is useful when you do not want to start processing the next
        schedule of a task until the dependents are done. For instance,
        if the task DROPs and recreates a table.
        :param session: SQLAlchemy ORM Session
        :type session: Session
        """
        task = self.task
        if not task.downstream_task_ids:
            return True
        ti = session.query(func.count(TaskInstance.task_id)).filter(
            TaskInstance.dag_id == self.dag_id,
            TaskInstance.task_id.in_(task.downstream_task_ids),
            TaskInstance.execution_date == self.execution_date,
            TaskInstance.state.in_([State.SKIPPED, State.SUCCESS]),
        )
        count = ti[0][0]
        return count == len(task.downstream_task_ids)
    @provide_session
    def get_previous_ti(