### 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
#
#
#
import json
from typing import Any, Dict, List, Optional
from airflow.models import BaseOperator
from airflow.providers.slack.hooks.slack import SlackHook
class SlackAPIOperator(BaseOperator):
    """
    Base Slack Operator
    The SlackAPIPostOperator is derived from this operator.
    In the future additional Slack API Operators will be derived from this class as well.
    Only one of `slack_conn_id` and `token` is required.
    :param slack_conn_id: :ref:`Slack connection id <howto/connection:slack>`
        which its password is Slack API token. Optional
    :type slack_conn_id: str
    :param token: Slack API token (https://api.slack.com/web). Optional
    :type token: str
    :param method: The Slack API Method to Call (https://api.slack.com/methods). Optional
    :type method: str
    :param api_params: API Method call parameters (https://api.slack.com/methods). Optional
    :type api_params: dict
    :param client_args: Slack Hook parameters. Optional. Check airflow.providers.slack.hooks.SlackHook
    :type client_args: dict
    """
    def __init__(