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
return self._execution_date
    @property
    def start_date(self) -> datetime:
        return self._start_date
    @property
    def end_date(self) -> datetime:
        return self._end_date
    @property
    def try_number(self) -> int:
        return self._try_number
    @property
    def state(self) -> str:
        return self._state
    @property
    def pool(self) -> str:
        return self._pool
    @property
    def priority_weight(self) -> Optional[int]:
        return self._priority_weight
    @property
    def queue(self) -> str:
        return self._queue
    @property
    def key(self) -> TaskInstanceKey:
        return self._key
    @property
    def executor_config(self):
        return self._executor_config
    @provide_session
    def construct_task_instance(self, session=None, lock_for_update=False) -> TaskInstance: