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
"""Filepath for TaskInstance"""
        iso = self.execution_date.isoformat()
        the_log = os.path.expanduser(conf.get('logging', 'BASE_LOG_FOLDER'))
        return f"{the_log}/{self.dag_id}/{self.task_id}/{iso}.log"
    @property
    def log_url(self):
        """Log URL for TaskInstance"""
        iso = quote(self.execution_date.isoformat())
        base_url = conf.get('webserver', 'BASE_URL')
        return base_url + f"/log?execution_date={iso}&task_id={self.task_id}&dag_id={self.dag_id}"
    @property
    def mark_success_url(self):
        """URL to mark TI success"""
        iso = quote(self.execution_date.isoformat())
        base_url = conf.get('webserver', 'BASE_URL')
        return base_url + (
            "/success"
            f"?task_id={self.task_id}"
            f"&dag_id={self.dag_id}"
            f"&execution_date={iso}"
            "&upstream=false"
            "&downstream=false"
        )
    @provide_session
    def current_state(self, session=None) -> str: