Remove file_annotation parameter from chat_postMessage

[mwbrooks]

In issue #1321 we've learned that the file_annotation parameter in the chat_postMessage method is has been removed from the Slack API and is no longer available.

I've confirmed that it's not included in Bolt for JavaScript or Bolt for Java. So, I think we only need to update the Python framework.

Category (place an x in each of the [ ])

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.webhook.WebhookClient (sync/async) (Incoming Webhook, response_url sender)
• slack_sdk.models (UI component builders)
• slack_sdk.oauth (OAuth Flow Utilities)
• slack_sdk.socket_mode (Socket Mode client)
• slack_sdk.audit_logs (Audit Logs API client)
• slack_sdk.scim (SCIM API client)
• slack_sdk.rtm (RTM client)
• slack_sdk.signature (Request Signature Verifier)

Requirements

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