docs: Fold FTS index updating into the upgrade step.
docs: Fold FTS index updating into the upgrade step.

On the Debian 10 -> 11 upgrade, the server is running Zulip 4.x, which
lets us pass `--audit-fts-indexes` to `upgrade-zulip-stage-2` rather
than run the command as a separate step.

andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,andersk/zulip,andersk/zulip