fix(deps): update dependency @google-cloud/pubsub to v2.15.1

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
@google-cloud/pubsub	2.12.0 -> 2.15.1

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Release Notes

googleapis/nodejs-pubsub

### [`v2.15.1`](https://togithub.com/googleapis/nodejs-pubsub/blob/master/CHANGELOG.md#​2151-httpswwwgithubcomgoogleapisnodejs-pubsubcomparev2150v2151-2021-06-22) [Compare Source](https://togithub.com/googleapis/nodejs-pubsub/compare/v2.14.0...v2.15.1) ### [`v2.14.0`](https://togithub.com/googleapis/nodejs-pubsub/blob/master/CHANGELOG.md#​2140-httpswwwgithubcomgoogleapisnodejs-pubsubcomparev2130v2140-2021-06-11) [Compare Source](https://togithub.com/googleapis/nodejs-pubsub/compare/v2.13.0...v2.14.0) ##### Features - upgrade [@​opentelemetry/api](https://togithub.com/opentelemetry/api) to version 0.20.0 ([#​1305](https://www.github.com/googleapis/nodejs-pubsub/issues/1305)) ([ed32369](https://www.github.com/googleapis/nodejs-pubsub/commit/ed32369ab4ccee5a8919f7353ed9eed7f3c1f6d8)) ### [`v2.13.0`](https://togithub.com/googleapis/nodejs-pubsub/blob/master/CHANGELOG.md#​2130-httpswwwgithubcomgoogleapisnodejs-pubsubcomparev2120v2130-2021-06-10) [Compare Source](https://togithub.com/googleapis/nodejs-pubsub/compare/v2.12.0...v2.13.0) ##### Features - expose list/get options for full/basic on schema methods ([#​1291](https://www.github.com/googleapis/nodejs-pubsub/issues/1291)) ([11f9612](https://www.github.com/googleapis/nodejs-pubsub/commit/11f961268429bb691be226aaffae6a88eeb9c600)) - schema library support ([#​1289](https://www.github.com/googleapis/nodejs-pubsub/issues/1289)) ([fd275e2](https://www.github.com/googleapis/nodejs-pubsub/commit/fd275e23fec89e6368a47caa1fdc194cbfc5d32b)) ##### Bug Fixes - GoogleAdsError missing using generator version after 1.3.0 ([#​1299](https://www.github.com/googleapis/nodejs-pubsub/issues/1299)) ([43a8fdb](https://www.github.com/googleapis/nodejs-pubsub/commit/43a8fdbbe2db24556101fb4e0bb29b3d95112ba0))

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