Upgrade GCP Libraries BOM to 26.87.0

[lqiu96]

This PR upgrades the Google Cloud Platform Libraries BOM from 26.85.0 to 26.87.0 and updates dependent/synchronized dependency versions:

• com.google.cloud:libraries-bom: 26.85.0 -> 26.87.0
• gax: 2.82.0 -> 2.84.0
• grpc: 1.81.0 -> 1.82.4
• netty: 4.1.132.Final -> 4.1.133.Final
• protobuf: 4.33.2 -> 4.33.6
• google-api-services-bigquery: v2-rev20260612-2.0.0 -> v2-rev20260731-2.0.0
• google-cloud-datastore: 3.2.0 -> 3.4.0
• sdks/java/container/license_scripts/dep_urls_java.yaml: updated libraries-bom license version
• Bigtable change stream cell mutations: aligned cell timestamp generation to millisecond granularity to satisfy Bigtable timestamp granularity constraints enforced by newer google-cloud-bigtable / google-cloud-bigtable-emulator.

[lqiu96]

Looks like the Google Maven Mirror doesn't have libraries-bom v26.87.0 yet: https://maven-central.storage-download.googleapis.com/maven2/com/google/cloud/libraries-bom/26.87.0/libraries-bom-26.87.0.pom

[github-actions]

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[lqiu96]

assign set of reviewers

[lqiu96]

Looks like the libraries-bom version is now in the maven mirror. Can we refresh the CIs?

[github-actions]

Assigning reviewers:

R: @chamikaramj for label java.
R: @liferoad for label build.
R: @igorbernstein2 for label bigtable.

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