deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.21.0 (#3905)
deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.21.0 (#3905)

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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) | `6.20.0` -> `6.21.0` | [![age](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.21.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.21.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.21.0/compatibility-slim/6.20.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/maven/com.google.cloud:google-cloud-spanner-bom/6.21.0/confidence-slim/6.20.0)](https://docs.renovatebot.com/merge-confidence/) |

---

### Release Notes

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

### [`v6.21.0`](https://togithub.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#&#8203;6210-httpsgithubcomgoogleapisjava-spannercomparev6200v6210-2022-03-08)

[Compare Source](https://togithub.com/googleapis/java-spanner/compare/v6.20.0...v6.21.0)

##### Features

-   parse query parameters in PostgreSQL query ([#&#8203;1732](https://togithub.com/googleapis/java-spanner/issues/1732)) ([7357ac6](https://togithub.com/googleapis/java-spanner/commit/7357ac6e3ddfdfee37e70343a970e7e63fb08bf2))
-   Track PG Adapter usage from user-agent headers ([#&#8203;1711](https://togithub.com/googleapis/java-spanner/issues/1711)) ([cb640ab](https://togithub.com/googleapis/java-spanner/commit/cb640abeb8ec9321136b86d5b54e620dba087080))

##### Bug Fixes

-   annotating some fields as REQUIRED ([#&#8203;1695](https://togithub.com/googleapis/java-spanner/issues/1695)) ([8b90b6c](https://togithub.com/googleapis/java-spanner/commit/8b90b6cce0fd36a1e3ca1c8e0c0f34661ab9c2a3))
-   catch ExecutionException for op.getName ([#&#8203;1729](https://togithub.com/googleapis/java-spanner/issues/1729)) ([8ea3ac0](https://togithub.com/googleapis/java-spanner/commit/8ea3ac086371beebd22f04c8c5f74beb8058e84f))
-   PostgreSQL supports newline in quoted literals and identifiers ([#&#8203;1731](https://togithub.com/googleapis/java-spanner/issues/1731)) ([f403d99](https://togithub.com/googleapis/java-spanner/commit/f403d99acd21db8d494855d71b5ec410164a5232))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-monitoring to v3.2.4 ([#&#8203;1719](https://togithub.com/googleapis/java-spanner/issues/1719)) ([20336cd](https://togithub.com/googleapis/java-spanner/commit/20336cd5d3307a48f968587212af38872dec5a50))
-   update dependency com.google.cloud:google-cloud-monitoring to v3.2.5 ([#&#8203;1727](https://togithub.com/googleapis/java-spanner/issues/1727)) ([92a9f14](https://togithub.com/googleapis/java-spanner/commit/92a9f148b8dcbd0ac7ca1ff0029ad7c09f577e40))
-   update dependency com.google.cloud:google-cloud-shared-dependencies to v2.8.0 ([#&#8203;1722](https://togithub.com/googleapis/java-spanner/issues/1722)) ([9704974](https://togithub.com/googleapis/java-spanner/commit/9704974a92f56886269e6cbcb1f74528fbe7e73f))
-   update dependency com.google.cloud:google-cloud-trace to v2.1.4 ([#&#8203;1728](https://togithub.com/googleapis/java-spanner/issues/1728)) ([d193a26](https://togithub.com/googleapis/java-spanner/commit/d193a26ec46df1b229103ec50c0db9b62d98507a))

</details>

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