chore(deps): update dependency google-api-core to v1.26.2 (#822)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-api-core](https://togithub.com/googleapis/python-api-core) | `==1.26.1` -> `==1.26.2` | [![age](https://badges.renovateapi.com/packages/pypi/google-api-core/1.26.2/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-api-core/1.26.2/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-api-core/1.26.2/compatibility-slim/1.26.1)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-api-core/1.26.2/confidence-slim/1.26.1)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-api-core</summary>

### [`v1.26.2`](https://togithub.com/googleapis/python-api-core/blob/master/CHANGELOG.md#&#8203;1262-httpswwwgithubcomgoogleapispython-api-corecomparev1261v1262-2021-03-23)

[Compare Source](https://togithub.com/googleapis/python-api-core/compare/v1.26.1...v1.26.2)

</details>

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packages/gapic-generator/requirements.txt

@@ -1,5 +1,5 @@
 click==7.1.2
-google-api-core==1.26.1
+google-api-core==1.26.2
 googleapis-common-protos==1.53.0
 jinja2==2.11.3
 MarkupSafe==1.1.1