docs: Update changelog with changes staged for 6.0.
docs: Update changelog with changes staged for 6.0.

We'll be merging a lot more features for the 6.0 release, but this
should cover everything merged since 5.0 and not backported to 5.x
and thus already released.

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