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
dag_id: str,  
        task_id: str,
        execution_date: datetime,
        mark_success: bool = False,
        ignore_all_deps: bool = False,
        ignore_depends_on_past: bool = False,
        ignore_task_deps: bool = False,
        ignore_ti_state: bool = False,
        local: bool = False,
        pickle_id: Optional[int] = None,
        file_path: Optional[str] = None,
        raw: bool = False,
        job_id: Optional[str] = None,
        pool: Optional[str] = None,
        cfg_path: Optional[str] = None,
    ) -> List[str]:
        """
        Generates the shell command required to execute this task instance.
        :param dag_id: DAG ID
        :type dag_id: str
        :param task_id: Task ID
        :type task_id: str
        :param execution_date: Execution date for the task
        :type execution_date: datetime