managed: document upgrade process with util scripts
Created by: bobheadxi
Documents https://github.com/sourcegraph/deploy-sourcegraph-managed/pull/23, see that PR for more detail. Closes https://github.com/sourcegraph/sourcegraph/issues/16228
Requires these patches to src-cli
: https://github.com/sourcegraph/src-cli/pull/414 , https://github.com/sourcegraph/src-cli/pull/413