release-24.1: changefeedccl: fix memory leak in deprecatedGcpPubsubClient

[blathers-crl]

Backport 1/1 commits from #121817 on behalf of @wenyihu6.

/cc @cockroachdb/release

---

Previously, deprecatedGcpPubsubClient did not close pubsub.Client properly
which could lead to memory leaks. This patch fixes the problem by invoking
pubsub.Client.Close() during deprecatedGcpPubsubClient.Close().

Epic: none
Release note: Fixed a slow memory leak in deprecated pubsub changefeeds which
can accumulate when restarting/cancelling many deprecated pubsub changefeeds.
The bug had been there since the deprecated pubsub was introduced in 22.1
(beta).

---

Release justification: (low-risk change) Fixed a bug which could lead to slow memory leak.

[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.
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Also, please add a brief release justification to the body of your PR to justify this
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[cockroach-teamcity]

This change is 

[wenyihu6]

The last push was to trigger local_roachtest which was stuck in CI.