chore(deps): update dependency protobuf to v3.14.0 (#699)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [protobuf](https://developers.google.com/protocol-buffers/) | minor | `==3.13.0` -> `==3.14.0` |

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requirements.txt

@@ -3,7 +3,7 @@ google-api-core==1.23.0
 googleapis-common-protos==1.52.0
 jinja2==2.11.2
 MarkupSafe==1.1.1
-protobuf==3.13.0
+protobuf==3.14.0
 pypandoc==1.5
 PyYAML==5.3.1
 dataclasses==0.6  # TODO(busunkim) remove when 3.6 support is dropped