chore(deps): update dependency backoff to v1.11.0 (#174)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [backoff](https://togithub.com/litl/backoff) | `==1.10.0` -> `==1.11.0` | [![age](https://badges.renovateapi.com/packages/pypi/backoff/1.11.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/backoff/1.11.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/backoff/1.11.0/compatibility-slim/1.10.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/backoff/1.11.0/confidence-slim/1.10.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>litl/backoff</summary>

### [`v1.11.0`](https://togithub.com/litl/backoff/blob/master/CHANGELOG.md#v1110-2021-07-12)

[Compare Source](https://togithub.com/litl/backoff/compare/v1.10.0...v1.11.0)

##### Changed

-   Configurable logging levels for backoff and giveup events
-   Minor documentation fixes

##### NOTE

THIS WILL BE THE FINAL PYTHON 2.7 COMPATIBLE RELEASE.

</details>

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packages/google-cloud-monitoring/samples/snippets/v3/cloud-client/requirements-test.txt

@@ -1,3 +1,3 @@
-backoff==1.10.0
+backoff==1.11.0
 pytest==6.2.4
 mock==4.0.3

packages/google-cloud-monitoring/samples/snippets/v3/uptime-check-client/requirements-test.txt

@@ -1,2 +1,2 @@
-backoff==1.10.0
+backoff==1.11.0
 pytest==6.2.4