release-24.3: cli: actually drain after decommission

[blathers-crl]

Backport 6/6 commits from #138732 on behalf of @tbg.

/cc @cockroachdb/release

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I do not know how this was ever supposed to work, but the old code can not have
been intentional: it created a drain client but then did not consume from it.
This had the effect of kicking off the drain, but ~immediately cancelling the
context on the goroutine carrying it out on the decommissioning node. This PR
actually waits for the drain to complete.

There is a related issue here, though. You can't drain a node that isn't live,
so attempting to decommission a node that's down will fail on the drain step.
This is certainly true as of this PR, but should have been true before as well.

Our docs¹ do not mention this rather large caveat at all, and it seems
strange anyway; if the node is down why would you let the failing drain get in
the way. Really the code ought to distinguish between the case of a live and
dead node and react accordingly - this is not something this PR achieves.

Fixes #138265.
Fixes #137240.

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Release justification:

Footnotes

1. https://www.cockroachlabs.com/docs/v24.3/node-shutdown?filters=decommission#remove-nodes
   Epic: none
   Release note (ops change): the node decommission cli command now waits
   until the target node is drained before marking it as fully
   decommissioned. Previously, it would start drain but not wait, leaving
   the target node briefly in a state where it would be unable to
   communicate with the cluster but would still accept client requests
   (which would then hang or hit unexpected errors). ↩

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[tbg]

I set a reminder to come back to this in a month or two.