chore(deps): update dependency google-cloud-bigquery to v2.21.0 (#226)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-bigquery](https://github.com/googleapis/python-bigquery) | `==2.20.0` -> `==2.21.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-bigquery/2.21.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-bigquery/2.21.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-bigquery/2.21.0/compatibility-slim/2.20.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-bigquery/2.21.0/confidence-slim/2.20.0)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

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

### [`v2.21.0`](https://github.com/googleapis/python-bigquery/blob/master/CHANGELOG.md#&#8203;2210-httpswwwgithubcomgoogleapispython-bigquerycomparev2200v2210-2021-07-12)

##### Features

-   Add max_results parameter to some of the `QueryJob` methods. ([#&#8203;698](https://github.com/googleapis/python-bigquery/issues/698)) ([2a9618f](https://github.com/googleapis/python-bigquery/commit/2a9618f4daaa4a014161e1a2f7376844eec9e8da))
-   Add support for decimal target types. ([#&#8203;735](https://github.com/googleapis/python-bigquery/issues/735)) ([7d2d3e9](https://github.com/googleapis/python-bigquery/commit/7d2d3e906a9eb161911a198fb925ad79de5df934))
-   Add support for table snapshots. ([#&#8203;740](https://github.com/googleapis/python-bigquery/issues/740)) ([ba86b2a](https://github.com/googleapis/python-bigquery/commit/ba86b2a6300ae5a9f3c803beeb42bda4c522e34c))
-   Enable unsetting policy tags on schema fields. ([#&#8203;703](https://github.com/googleapis/python-bigquery/issues/703)) ([18bb443](https://github.com/googleapis/python-bigquery/commit/18bb443c7acd0a75dcb57d9aebe38b2d734ff8c7))
-   Make it easier to disable best-effort deduplication with streaming inserts. ([#&#8203;734](https://github.com/googleapis/python-bigquery/issues/734)) ([1246da8](https://github.com/googleapis/python-bigquery/commit/1246da86b78b03ca1aa2c45ec71649e294cfb2f1))
-   Support passing struct data to the DB API. ([#&#8203;718](https://github.com/googleapis/python-bigquery/issues/718)) ([38b3ef9](https://github.com/googleapis/python-bigquery/commit/38b3ef96c3dedc139b84f0ff06885141ae7ce78c))

##### Bug Fixes

-   Inserting non-finite floats with `insert_rows()`. ([#&#8203;728](https://github.com/googleapis/python-bigquery/issues/728)) ([d047419](https://github.com/googleapis/python-bigquery/commit/d047419879e807e123296da2eee89a5253050166))
-   Use `pandas` function to check for `NaN`. ([#&#8203;750](https://github.com/googleapis/python-bigquery/issues/750)) ([67bc5fb](https://github.com/googleapis/python-bigquery/commit/67bc5fbd306be7cdffd216f3791d4024acfa95b3))

##### Documentation

-   Add docs for all enums in module. ([#&#8203;745](https://github.com/googleapis/python-bigquery/issues/745)) ([145944f](https://github.com/googleapis/python-bigquery/commit/145944f24fedc4d739687399a8309f9d51d43dfd))
-   Omit mention of Python 2.7 in `CONTRIBUTING.rst`. ([#&#8203;706](https://github.com/googleapis/python-bigquery/issues/706)) ([27d6839](https://github.com/googleapis/python-bigquery/commit/27d6839ee8a40909e4199cfa0da8b6b64705b2e9))

</details>

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asset/snippets/snippets/requirements.txt

@@ -2,4 +2,4 @@ google-cloud-storage==1.40.0
 google-cloud-asset==3.2.0
 google-cloud-resource-manager==0.30.3
 google-cloud-pubsub==2.6.1
-google-cloud-bigquery==2.20.0
+google-cloud-bigquery==2.21.0