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
"""Remake the key by subtracting 1 from try number to match in memory information"""
        return TaskInstanceKey(self.dag_id, self.task_id, self.execution_date, max(1, self.try_number - 1))
    def with_try_number(self, try_number: int) -> 'TaskInstanceKey':
        """Returns TaskInstanceKey with provided ``try_number``"""
        return TaskInstanceKey(self.dag_id, self.task_id, self.execution_date, try_number)
class TaskInstance(Base, LoggingMixin):  
    """
    Task instances store the state of a task instance. This table is the
    authority and single source of truth around what tasks have run and the
    state they are in.
    The SqlAlchemy model doesn't have a SqlAlchemy foreign key to the task or
    dag model deliberately to have more control over transactions.
    Database transactions on this table should insure double triggers and
    any confusion around what task instances are or aren't ready to run
    even while multiple schedulers may be firing task instances.
    """
    __tablename__ = "task_instance"
    task_id = Column(String(ID_LEN, **COLLATION_ARGS), primary_key=True)
    dag_id = Column(String(ID_LEN, **COLLATION_ARGS), primary_key=True)
    execution_date = Column(UtcDateTime, primary_key=True)
    start_date = Column(UtcDateTime)
    end_date = Column(UtcDateTime)
    duration = Column(Float)
    state = Column(String(20))
    _try_number = Column('try_number', Integer, default=0)
    max_tries = Column(Integer)
    hostname = Column(String(1000))