Exactly one of Topic or Subscription must be set in pubsubio.go

[TanuSharma2511]

It resolves #35295

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[github-actions]

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[TanuSharma2511]

Run Go PreCommit

[TanuSharma2511]

R: @lostluck

[github-actions]

Assigning reviewers:

R: @shunping for label go.

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[lostluck]

Please add an entry in CHANGES.md about the breaking change as well.

[lostluck]

waiting on author

[TanuSharma2511]

@lostluck I have addressed the comments.

[lostluck]

Just the one typo, and then I'll merge it in. Thank you kindly!