Update Fluent forced colors snapshots now that hideSendBox hides the send box

The forced colors tests from #5907 share pages that render Web Chat with
`styleOptions: { hideSendBox: true }`. Their Fluent snapshots were captured
before the Fluent theme honored `hideSendBox`, so they still showed the send
box. With the send box hidden, the messages sit 90px lower; nothing else
changes.

Author: Eugene Olonov