fix(deps): update dependency @google-cloud/pubsub to v2.18.4

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@google-cloud/pubsub	2.18.3 -> 2.18.4

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Release Notes

googleapis/nodejs-pubsub

### [`v2.18.4`](https://togithub.com/googleapis/nodejs-pubsub/blob/HEAD/CHANGELOG.md#​2184-httpswwwgithubcomgoogleapisnodejs-pubsubcomparev2183v2184-2021-12-09) [Compare Source](https://togithub.com/googleapis/nodejs-pubsub/compare/v2.18.3...v2.18.4)

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