chore(deps): update dependency google-cloud-pubsub to v2.6.0 (#168)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-pubsub](https://togithub.com/googleapis/python-pubsub) | `==2.5.0` -> `==2.6.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.0/compatibility-slim/2.5.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-pubsub/2.6.0/confidence-slim/2.5.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-pubsub</summary>

### [`v2.6.0`](https://togithub.com/googleapis/python-pubsub/blob/master/CHANGELOG.md#&#8203;260-httpswwwgithubcomgoogleapispython-pubsubcomparev250v260-2021-06-17)

[Compare Source](https://togithub.com/googleapis/python-pubsub/compare/v2.5.0...v2.6.0)

##### Features

-   support customizable retry and timeout settings on the publisher client ([#&#8203;299](https://www.github.com/googleapis/python-pubsub/issues/299)) ([7597604](https://www.github.com/googleapis/python-pubsub/commit/7597604b41fa3a1e9bf34addc35c8647dde007cc))

##### Bug Fixes

-   ACK deadline set for received messages can be too low  ([#&#8203;416](https://www.github.com/googleapis/python-pubsub/issues/416)) ([e907f6e](https://www.github.com/googleapis/python-pubsub/commit/e907f6e05f59f64a3b08df3304e92ec960997be6))
-   threads can skip the line in publisher flow controller ([#&#8203;422](https://www.github.com/googleapis/python-pubsub/issues/422)) ([ef89f55](https://www.github.com/googleapis/python-pubsub/commit/ef89f55a41044e9ad26b91132b4b1be9c7b2c127))

##### Documentation

-   block until the streaming pull shuts down ([#&#8203;424](https://www.github.com/googleapis/python-pubsub/issues/424)) ([d0d0b70](https://www.github.com/googleapis/python-pubsub/commit/d0d0b704642df8dee893d3f585aeb666e19696fb))
-   explain that future.cancel() is non-blocking ([#&#8203;420](https://www.github.com/googleapis/python-pubsub/issues/420)) ([c825789](https://www.github.com/googleapis/python-pubsub/commit/c825789bdff310f44cbb132a723e99d1e6331d8f))

</details>

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samples/snippets/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-pubsub==2.5.0
+google-cloud-pubsub==2.6.0
 google-cloud-securitycenter==1.3.1