cli: fully and properly drain target node of decommission

[tbg]

With this patch, at the end of decommissioning, we call the drain step as we
would for ./cockroach node drain:

[...]
.....
id	is_live	replicas	is_decommissioning	membership	is_draining	readiness	blocking_ranges
1	true	2	true	decommissioning	false	ready	0
.....
id	is_live	replicas	is_decommissioning	membership	is_draining	readiness	blocking_ranges
1	true	1	true	decommissioning	false	ready	0
......
id	is_live	replicas	is_decommissioning	membership	is_draining	readiness	blocking_ranges
1	true	0	true	decommissioning	false	ready	0
draining node n2
node is draining... remaining: 26
node is draining... remaining: 0 (complete)
node n2 drained successfully

No more data reported on target nodes. Please verify cluster health before removing the nodes.

In particular, note how the first invocation returns a RemainingIndicator of
26, so before this patch, we had initiated draining, but it hadn't fully completed.

I thought for a while that this could explain #140774, i.e. that #138732 was
insufficient as it did not guarantee that the node had actually drained fully
by the time it was marked as fully decommissioned and the node decommission
had returned. But I found that fully draining did not fix the test, and
ultimately tracked the issue down to a test infra problem. Still, this PR is
a good change, that brings the drain experience in decommission on par with
the standalone CLI.

See #140774.

I verified that the modified decommission/drains roachtest passes via

./pkg/cmd/roachtest/roachstress.sh -l -c 1 decommission/drains/alive

Touches #140774.
Touches #139411.
Touches #139413.

PR #138732 already fixed most of the drain issues, but since the
decommissioning process still went ahead and shut the node out
from the cluster, SQL connections that drain was still waiting
for would likely hit errors (since the gateway node would not
be able to connect to the rest of the cluster any more due to
having been flipped to fully decommissioned). So there's a new
release note for the improvement in this PR, which avoids that.

Release note (bug fix): previously, a node that was drained as part
of decommissioning may have interrupted SQL connections that were
still active during drain (and for which drain would have been
expected to wait).
Epic: None

[cockroach-teamcity]

This change is