Update handbook references to old Slack channel names
Created by: dadlerj
Context: https://sourcegraph.slack.com/archives/C02FSM7DU/p1619649670475800 Additional context: https://github.com/sourcegraph/about/pull/3023
I've tagged affected team leads/members, and I'd love any fixes if anyone sees any issues (@virginiaulrich in particular given the number of changes to support pages), but I plan to merge tomorrow evening PT given the channel name changes have already happened.