docs: Update upgrading docs

Created by: efritz

This closes https://github.com/sourcegraph/sourcegraph/issues/14604.

This updates the admin docs for updating Sourcegraph. This moves some of the server-specific data from the index page into its page so that we don't muck up the flow for other environments.

The main change here is a note in the upgrade from 3.20 -> 3.21, which calls out that a background process is migrating data and you must see a migration complete message before moving on to 3.22 (which will assume that no more data exists on the bundle manager disk as part of our cleanup transition).

Once 3.22 is released, I will also update docs for the 3.21.? -> 3.22 migration which will re-iterate the same thing: if you hadn't seen this log or verified this property, you will lose data.

Other migration paths that do require some amount of time be spent in a particular version should follow this same pattern (unless someone on distribution has a rebuttal).

Note to @uwedeportivo that this is a bit more information you should add to https://github.com/sourcegraph/sourcegraph/issues/14347.