chore(deps): update dependency google-cloud-bigtable to v1.5.1 (GoogleCloudPlatform#4818)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-bigtable](https://togithub.com/googleapis/python-bigtable) | patch | `==1.5.0` -> `==1.5.1` |

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### Release Notes

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

### [`v1.5.1`](https://togithub.com/googleapis/python-bigtable/blob/master/CHANGELOG.md#&#8203;151-httpswwwgithubcomgoogleapispython-bigtablecomparev150v151-2020-10-06)

[Compare Source](https://togithub.com/googleapis/python-bigtable/compare/v1.5.0...v1.5.1)

</details>

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functions/bigtable/requirements.txt

@@ -1 +1 @@
-google-cloud-bigtable==1.5.0
+google-cloud-bigtable==1.5.1