### Apache Airflow version

2.2.0

### Operating System

Debian buster

### Versions of Apache Airflow Providers

_No response_

### Deployment

Astronomer

### Deployment details

astro dev start

### What happened

When I have tried to duplicate a connection first time it has created a connection with hello_copy1 and duplicating the same connection second time giving me an error of 
```Connection hello_copy2 can't be added. Integrity error, probably unique constraint.```




### What you expected to happen

It should create a connection with name hello_copy3 or the error message should be more user friendly.

### How to reproduce

https://github.com/apache/airflow/pull/15574#issuecomment-912438705

### Anything else

Suggested Error 
```
Connection hello_copy2 can't be added because it already exists.
```
or
Change the number logic in _copyN so the new name will be always unique.


### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/www/views.py
@expose('/task_stats', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
            (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        ]
    )
    @provide_session
    def task_stats(self, session=None):
        """Task Statistics"""
        allowed_dag_ids = current_app.appbuilder.sm.get_accessible_dag_ids(g.user)
        if not allowed_dag_ids:
            return wwwutils.json_response({})
        
        selected_dag_ids = {unquote(dag_id) for dag_id in request.form.getlist('dag_ids') if dag_id}
        if selected_dag_ids:
            filter_dag_ids = selected_dag_ids.intersection(allowed_dag_ids)
        else:
            filter_dag_ids = allowed_dag_ids
        running_dag_run_query_result = (
            session.query(DagRun.dag_id, DagRun.run_id)
            .join(DagModel, DagModel.dag_id == DagRun.dag_id)
            .filter(DagRun.state == State.RUNNING, DagModel.is_active)
        )
        running_dag_run_query_result = running_dag_run_query_result.filter(DagRun.dag_id.in_(filter_dag_ids))
        running_dag_run_query_result = running_dag_run_query_result.subquery('running_dag_run')
        
        running_task_instance_query_result = session.query(
            TaskInstance.dag_id.label('dag_id'), TaskInstance.state.label('state')