deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.56.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.cloud:google-cloud-spanner-bom	6.55.0 -> 6.56.0

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Release Notes

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

v6.56.0

Compare Source

Features

• Add autoscaling config in the instance to support autoscaling in systests (#​2756) (99ae565)
• Add support for Directed Read options (#​2766) (26c6c63)
• Update OwlBot.yaml file to pull autogenerated executor code (#​2754) (20562d4)

Dependencies

• Update dependency com.google.cloud:google-cloud-shared-dependencies to v3.21.0 (#​2772) (173f520)

Documentation

• Samples and tests for auto-generated createDatabase and createInstance APIs. (#​2764) (74a586f)

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