Migrates to Slack file upload v2 API call

[mvilanova]

Changes

• Updated the Slack file upload method from files_upload to files_upload_v2 in the SlackConversationPlugin class
• Changed parameter name from channels to channel to match the new API requirements
• Modified the channel value to use the conversation ID from signal_response.get("id") instead of using conversation_id
• Added a comment explaining that we need the conversation ID, not the name
• Removed the filetype="json" parameter as it's no longer needed in the v2 API

This change ensures compatibility with Slack's updated file upload API.