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.refresh_from_task(task, pool_override=pool)
        self.test_mode = test_mode
        self.refresh_from_db(session=session, lock_for_update=True)
        self.job_id = job_id
        self.hostname = get_hostname()
        if not ignore_all_deps and not ignore_ti_state and self.state == State.SUCCESS:
            Stats.incr('previously_succeeded', 1, 1)
        
        hr_line_break = "\n" + ("-" * 80)  
        if not mark_success:
            
            
            non_requeueable_dep_context = DepContext(
                deps=RUNNING_DEPS - REQUEUEABLE_DEPS,
                ignore_all_deps=ignore_all_deps,
                ignore_ti_state=ignore_ti_state,
                ignore_depends_on_past=ignore_depends_on_past,
                ignore_task_deps=ignore_task_deps,
            )
            if not self.are_dependencies_met(
                dep_context=non_requeueable_dep_context, session=session, verbose=True
            ):
                session.commit()
                return False
            
            
            
            
            
            self.start_date = timezone.utcnow()