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 a tuple that identifies the task instance uniquely"""
        return TaskInstanceKey(self.dag_id, self.task_id, self.execution_date, self.try_number)
    @provide_session
    def set_state(self, state: str, session=None):
        """
        Set TaskInstance state.
        :param state: State to set for the TI
        :type state: str
        :param session: SQLAlchemy ORM Session
        :type session: Session
        """
        current_time = timezone.utcnow()
        self.log.debug("Setting task state for %s to %s", self, state)
        self.state = state
        self.start_date = self.start_date or current_time
        if self.state in State.finished or self.state == State.UP_FOR_RETRY:
            self.end_date = self.end_date or current_time
            self.duration = (self.end_date - self.start_date).total_seconds()
        session.merge(self)
    @property
    def is_premature(self):