docs: Document upgrade steps from buster to bullseye.
docs: Document upgrade steps from buster to bullseye.

Fixes #17863.

zulip/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip