Add deprecation warning to deprecated methods

[omerkarpas]

example below from slack_sdk/web/client.py

why not delete them are at least add docstr or a warning message to the log these are deprecated!

    # --------------------------
    # Deprecated: channels.*
    # You can use conversations.* APIs instead.
    # https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api
    # --------------------------

    def channels_archive(
        self,
        *,
        channel: str,
        **kwargs,
    ) -> SlackResponse:
        """Archives a channel."""
        kwargs.update({"channel": channel})
        kwargs = _remove_none_values(kwargs)
        return self.api_call("channels.archive", json=kwargs)

[filmaj]

Hey @omerkarpas , thanks for the suggestion! I think adding a deprecation warning to these old API methods is a good idea.

Feel free to send a PR! 🙇

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