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
self._dag_id: str = ti.dag_id
        self._task_id: str = ti.task_id
        self._execution_date: datetime = ti.execution_date
        self._start_date: datetime = ti.start_date
        self._end_date: datetime = ti.end_date
        self._try_number: int = ti.try_number
        self._state: str = ti.state
        self._executor_config: Any = ti.executor_config
        self._run_as_user: Optional[str] = None
        if hasattr(ti, 'run_as_user'):
            self._run_as_user = ti.run_as_user
        self._pool: str = ti.pool
        self._priority_weight: Optional[int] = None
        if hasattr(ti, 'priority_weight'):
            self._priority_weight = ti.priority_weight
        self._queue: str = ti.queue
        self._key = ti.key
    
    @property
    def dag_id(self) -> str:
        return self._dag_id
    @property
    def task_id(self) -> str:
        return self._task_id
    @property
    def execution_date(self) -> datetime: