deps: update dependency gcp-releasetool to v1.8.8 (googleapis#719)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| gcp-releasetool | `==1.8.7` -> `==1.8.8` | [![age](https://badges.renovateapi.com/packages/pypi/gcp-releasetool/1.8.8/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/gcp-releasetool/1.8.8/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/gcp-releasetool/1.8.8/compatibility-slim/1.8.7)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/gcp-releasetool/1.8.8/confidence-slim/1.8.7)](https://docs.renovatebot.com/merge-confidence/) |

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