### 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
"""
    Posts messages to a slack channel
    Examples:
    .. code-block:: python
        slack = SlackAPIPostOperator(
            task_id="post_hello",
            dag=dag,
            token="XXX",
            text="hello there!",
            channel="#random",
        )
    :param channel: channel in which to post message on slack name (#general) or
        ID (C12318391). (templated)
    :type channel: str
    :param username: Username that airflow will be posting to Slack as. (templated)
    :type username: str
    :param text: message to send to slack. (templated)
    :type text: str
    :param icon_url: url to icon used for this message
    :type icon_url: str
    :param attachments: extra formatting details. (templated)
        - see https://api.slack.com/docs/attachments.
    :type attachments: list of hashes
    :param blocks: extra block layouts. (templated)
        - see https://api.slack.com/reference/block-kit/blocks.
    :type blocks: list of hashes
    """
    template_fields = ('username', 'text', 'attachments', 'blocks', 'channel')
    ui_color = '#FFBA40'
    def __init__(