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

[![WhiteSource Renovate](https://app.renovatebot.com/images/banner.svg)](https://renovatebot.com)

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/) |

---

### Configuration

📅 **Schedule**: At any time (no schedule defined).

🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻️ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box.

---

This PR has been generated by [WhiteSource Renovate](https://renovate.whitesourcesoftware.com). View repository job log [here](https://app.renovatebot.com/dashboard#github/googleapis/python-bigquery).

Author: renovate-bot

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'