### Apache Airflow Provider(s)

slack

### Versions of Apache Airflow Providers

apache-airflow-providers-slack==3.0.0

### Apache Airflow version

2.1.4 (latest released)

### Operating System

Debian GNU/Linux 11 (bullseye)

### Deployment

Other Docker-based deployment

### Deployment details

_No response_

### What happened

Using a template string for `SlackAPIFileOperator.filename` doesn't apply the templating.

### What you expected to happen

`SlackAPIFileOperator.filrname` should work with a template string.

### How to reproduce

Use a template string when using `SlackAPIFileOperator`.

### Anything else

Re: PR: https://github.com/apache/airflow/pull/17400/files
In commit: https://github.com/apache/airflow/pull/17400/commits/bbd11de2f1d37e9e2f07e5f9b4d331bf94ef6b97

`filename` was removed from `template_fields`.  I believe this was because `filename` was removed as a parameter in favour of `file`.  In a later commit `file` was renamed to `filename` but the `template_fields` was not put back.  The documentation still states that it's a templated field.

### Are you willing to submit PR?

- [X] 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/providers/slack/operators/slack.py
"""
    Send a file to a slack channel
    Examples:
    .. code-block:: python
        # Send file with filename and filetype
        with open("test.txt", "rb") as file:
            slack = SlackAPIFileOperator(
                task_id="slack_file_upload",
                dag=dag,
                slack_conn_id="slack",
                channel="#general",
                initial_comment="Hello World!",
                file="/files/dags/test.txt",
                filetype="txt",
            )
        # Send file content
        slack = SlackAPIFileOperator(
            task_id="slack_file_upload",
            dag=dag,
            slack_conn_id="slack",
            channel="#general",
            initial_comment="Hello World!",
            content="file content in txt",
        )
    :param channel: channel in which to sent file on slack name (templated)
    :type channel: str