chore(deps): update dependency grpcio to v1.38.0 (#674)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [grpcio](https://grpc.io) | `==1.37.1` -> `==1.38.0` | [![age](https://badges.renovateapi.com/packages/pypi/grpcio/1.38.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/grpcio/1.38.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/grpcio/1.38.0/compatibility-slim/1.37.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/grpcio/1.38.0/confidence-slim/1.37.1)](https://docs.renovatebot.com/merge-confidence/) |

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samples/snippets/requirements.txt

@@ -1,7 +1,7 @@
 google-cloud-bigquery==2.16.1
 google-cloud-bigquery-storage==2.4.0
 google-auth-oauthlib==0.4.4
-grpcio==1.37.1
+grpcio==1.38.0
 ipython==7.16.1; python_version < '3.7'
 ipython==7.17.0; python_version >= '3.7'
 matplotlib==3.3.4; python_version < '3.7'