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
"""Fetch rendered template fields from DB"""
        from airflow.models.renderedtifields import RenderedTaskInstanceFields
        rendered_task_instance_fields = RenderedTaskInstanceFields.get_templated_fields(self)
        if rendered_task_instance_fields:
            for field_name, rendered_value in rendered_task_instance_fields.items():
                setattr(self.task, field_name, rendered_value)
        else:
            try:
                self.render_templates()
            except (TemplateAssertionError, UndefinedError) as e:
                raise AirflowException(
                    "Webserver does not have access to User-defined Macros or Filters "
                    "when Dag Serialization is enabled. Hence for the task that have not yet "
                    "started running, please use 'airflow tasks render' for debugging the "
                    "rendering of template_fields."
                ) from e
    def get_rendered_k8s_spec(self):
        """Fetch rendered template fields from DB"""
        from airflow.models.renderedtifields import RenderedTaskInstanceFields
        rendered_k8s_spec = RenderedTaskInstanceFields.get_k8s_pod_yaml(self)
        if not rendered_k8s_spec:
            try:
                rendered_k8s_spec = self.render_k8s_pod_yaml()
            except (TemplateAssertionError, UndefinedError) as e:
                raise AirflowException(f"Unable to render a k8s spec for this taskinstance: {e}") from e
        return rendered_k8s_spec
    def overwrite_params_with_dag_run_conf(self, params, dag_run):