Update the upgrade guide to mention that sidekiq should be stopped after the web process

[matthewford]

mastodon/mastodon#20250

At the moment, it suggests restarting sidekiq; not sure if it was mentioned in the release notes to clear the queue, but doing this (e.g from RC1 to RC2) if there is a change to the workers, it will mean jobs in flight are trying to load old worker code