fix(deps): update dependency @google-cloud/pubsub to v2.7.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@google-cloud/pubsub	2.6.0 -> 2.7.0

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Release Notes

googleapis/nodejs-pubsub

### [`v2.7.0`](https://togithub.com/googleapis/nodejs-pubsub/blob/master/CHANGELOG.md#​270-httpswwwgithubcomgoogleapisnodejs-pubsubcomparev260v270-2020-11-30) [Compare Source](https://togithub.com/googleapis/nodejs-pubsub/compare/v2.6.0...v2.7.0) ##### Features - Enable server side flow control by default with the option to turn it off ([#​1147](https://www.github.com/googleapis/nodejs-pubsub/issues/1147)) ([a9c7e0b](https://www.github.com/googleapis/nodejs-pubsub/commit/a9c7e0b216a0c228793eeead150cc21b16bd2dcb)) - update generator to a new version ([#​1157](https://www.github.com/googleapis/nodejs-pubsub/issues/1157)) ([3b1e99e](https://www.github.com/googleapis/nodejs-pubsub/commit/3b1e99ebde72653a1fa0317820d1218e1c04eb8f)) ##### Bug Fixes - do not modify options object, use defaultScopes ([#​1148](https://www.github.com/googleapis/nodejs-pubsub/issues/1148)) ([4745c10](https://www.github.com/googleapis/nodejs-pubsub/commit/4745c10eb062b1816516ce79cec496b388e48b2b)) - node 14+ changes how multiple destroy() calls work ([#​1153](https://www.github.com/googleapis/nodejs-pubsub/issues/1153)) ([e421749](https://www.github.com/googleapis/nodejs-pubsub/commit/e42174987ec4669b31882943606c871c9b6c08d7))

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