deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.88.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.cloud:google-cloud-spanner-bom	6.87.0 -> 6.88.0

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Release Notes

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

v6.88.0

Compare Source

Features

• Add a last field in the PartialResultSet (7c714be)
• Automatically set default sequence kind in JDBC and PGAdapter (#​3658) (e8abf33)
• Default authentication support for external hosts (#​3656) (ace11d5)
• spanner: A new enum IsolationLevel is added (3fd33ba)
• spanner: Add instance partitions field in backup proto (3fd33ba)

Bug Fixes

• deps: Update the Java code generator (gapic-generator-java) to 2.54.0 (57497ad)

Dependencies

• Update dependency com.google.cloud:sdk-platform-java-config to v3.44.0 (#​3665) (3543548)

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[dpebot]

/gcbrun

[dpebot]

/gcbrun