deps: update dependency com.google.protobuf:protobuf-bom to v3.15.8 (#418)
deps: update dependency com.google.protobuf:protobuf-bom to v3.15.8 (#418)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.protobuf:protobuf-bom](https://developers.google.com/protocol-buffers/) ([source](https://togithub.com/protocolbuffers/protobuf)) | `3.15.7` -> `3.15.8` | [![age](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.8/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.8/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.8/compatibility-slim/3.15.7)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.protobuf:protobuf-bom/3.15.8/confidence-slim/3.15.7)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>protocolbuffers/protobuf</summary>

### [`v3.15.8`](https://togithub.com/protocolbuffers/protobuf/releases/v3.15.8)

[Compare Source](https://togithub.com/protocolbuffers/protobuf/compare/v3.15.7...v3.15.8)

### Ruby

-   Fixed memory leak of Ruby arena objects ([#&#8203;8461](https://togithub.com/protocolbuffers/protobuf/issues/8461))

</details>

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