chore: Ignore Websocket MessageCount

[alexandreferris]

• Bring back MessageCount("message_count") response class
• Listen in websocket frame but ignore ACK

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• The PR Title

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What's new in this PR?

Issues

Despite not needing to send an ACK for MessageCount("message_count") from the websocket anymore we still need to receive it (but ignore) until wireapp/wire-server#4631 is merged.

Solutions

• Update backend version to v9
• Bring back MessageCount
• Listen to MessageCount from the websocket but ignore/don't send an ACK