deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.28.0 (#4912)
deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.28.0 (#4912)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [com.google.cloud:google-cloud-spanner-bom](https://togithub.com/googleapis/java-spanner) | `6.27.0` -> `6.28.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.28.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.28.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.28.0/compatibility-slim/6.27.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.28.0/confidence-slim/6.27.0)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

<details>
<summary>googleapis/java-spanner</summary>

### [`v6.28.0`](https://togithub.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#&#8203;6280-httpsgithubcomgoogleapisjava-spannercomparev6270v6280-2022-08-11)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.27.0...v6.28.0)

##### Features

-   Add ListDatabaseRoles API to support role based access control ([cb13534](https://togithub.com/googleapis/java-spanner/commit/cb13534d7ca2e1b581cb4551d0f95834fbf7b640))
-   support multiple PostgreSQL transaction options ([#&#8203;1949](https://togithub.com/googleapis/java-spanner/issues/1949)) ([8b99f30](https://togithub.com/googleapis/java-spanner/commit/8b99f30285e4ef68376aa9bfc11617f74e110bf2))

##### Bug Fixes

-   target new spanner db admin service config ([#&#8203;1956](https://togithub.com/googleapis/java-spanner/issues/1956)) ([cb13534](https://togithub.com/googleapis/java-spanner/commit/cb13534d7ca2e1b581cb4551d0f95834fbf7b640))
-   Use the key instead of the value to verify the number of channels created in ChannelUsageTest. ([#&#8203;1965](https://togithub.com/googleapis/java-spanner/issues/1965)) ([ea329bb](https://togithub.com/googleapis/java-spanner/commit/ea329bb57b343c58bab2680b0c9412e51522b90b))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-monitoring to v3.3.6 ([#&#8203;1962](https://togithub.com/googleapis/java-spanner/issues/1962)) ([5bb9844](https://togithub.com/googleapis/java-spanner/commit/5bb98441d65ba462c49810f980770406df8ca127))
-   update dependency com.google.cloud:google-cloud-shared-dependencies to v3 ([#&#8203;1960](https://togithub.com/googleapis/java-spanner/issues/1960)) ([327b5f0](https://togithub.com/googleapis/java-spanner/commit/327b5f069f8fe4625be49c258c721a4db5fb0f6e))
-   update dependency org.junit.vintage:junit-vintage-engine to v5.9.0 ([#&#8203;1959](https://togithub.com/googleapis/java-spanner/issues/1959)) ([f908626](https://togithub.com/googleapis/java-spanner/commit/f90862667613280a8c7a2901ba4b5940b0647eb2))

</details>

---

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