deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.81.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.cloud:google-cloud-spanner-bom	6.80.1 -> 6.81.0

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Release Notes

googleapis/java-spanner (com.google.cloud:google-cloud-spanner-bom)

### [`v6.81.0`](https://redirect.github.com/googleapis/java-spanner/blob/HEAD/CHANGELOG.md#6810-2024-11-01) [Compare Source](https://redirect.github.com/googleapis/java-spanner/compare/v6.80.1...v6.81.0) ##### Features - Client built in metrics ([#​3408](https://redirect.github.com/googleapis/java-spanner/issues/3408)) ([6a36103](https://redirect.github.com/googleapis/java-spanner/commit/6a3610379d1d0eee741d5ef4b30e811ff5a67bc0)) ##### Dependencies - Update dependency com.google.cloud:google-cloud-monitoring to v3.54.0 ([#​3439](https://redirect.github.com/googleapis/java-spanner/issues/3439)) ([cdec63f](https://redirect.github.com/googleapis/java-spanner/commit/cdec63f84ef9b615adf19e4611b2dc223eec687b))

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