**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): N/A

**Environment**: 

- **Cloud provider or hardware configuration**: local/aws
- **OS** (e.g. from /etc/os-release): Ubuntu 18.04.5 LTS
- **Kernel** (e.g. `uname -a`): 5.4.0-1032-aws
- **Install tools**: pip
- **Others**:

**What happened**:
I did a fresh Airflow 2.0.0 install. With this version, when I manually trigger a DAG, Airflow skips the next scheduled run.
<!-- (please include exact error messages if you can) -->

**What you expected to happen**:
Manual runs do not interfere with the scheduled runs prior to Airflow 2. 
<!-- What do you think went wrong? -->

**How to reproduce it**:
Create a simple hourly DAG. After enabling it and the initial run, run it manually. It shall skip the next hour. Below is an example, where the manual run with execution time of 08:17 causes the scheduled run with execution time of 08:00 to skip. 
![image](https://user-images.githubusercontent.com/26160471/103457719-c7193480-4d12-11eb-82cb-42efaedc9ef4.png)


airflow/models/dag.py
"""Returns the latest date for which at least one dag run exists"""
        return session.query(func.max(DagRun.execution_date)).filter(DagRun.dag_id == self.dag_id).scalar()
    @property
    def latest_execution_date(self):
        """This attribute is deprecated. Please use `airflow.models.DAG.get_latest_execution_date` method."""
        warnings.warn(
            "This attribute is deprecated. Please use `airflow.models.DAG.get_latest_execution_date` method.",
            DeprecationWarning,
            stacklevel=2,
        )
        return self.get_latest_execution_date()
    @property
    def subdags(self):
        """Returns a list of the subdag objects associated to this DAG"""
        
        from airflow.operators.subdag import SubDagOperator
        subdag_lst = []
        for task in self.tasks:
            if (
                isinstance(task, SubDagOperator)
                or
                
                type(task).__name__ == 'SubDagOperator'
                or task.task_type == 'SubDagOperator'
            ):
                subdag_lst.append(task.subdag)
                subdag_lst += task.subdag.subdags
        return subdag_lst
    def resolve_template_files(self):