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
if self.state == State.UP_FOR_RESCHEDULE:
                task_reschedule: TR = TR.query_for_task_instance(self, session=session).first()
                if task_reschedule:
                    self.start_date = task_reschedule.start_date
            
            
            
            dep_context = DepContext(
                deps=REQUEUEABLE_DEPS,
                ignore_all_deps=ignore_all_deps,
                ignore_depends_on_past=ignore_depends_on_past,
                ignore_task_deps=ignore_task_deps,
                ignore_ti_state=ignore_ti_state,
            )
            if not self.are_dependencies_met(dep_context=dep_context, session=session, verbose=True):
                self.state = State.NONE
                self.log.warning(hr_line_break)
                self.log.warning(
                    "Rescheduling due to concurrency limits reached "
                    "at task runtime. Attempt %s of "
                    "%s. State set to NONE.",
                    self.try_number,
                    self.max_tries + 1,
                )
                self.log.warning(hr_line_break)
                self.queued_dttm = timezone.utcnow()
                session.merge(self)
                session.commit()
                return False