deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.87.0#1925

gcf-merge-on-green[bot] merged 2 commits intogoogleapis:maingoogleapis/java-spanner-jdbc:mainfrom

renovate-bot:renovate/com.google.cloud-google-cloud-spanner-bom-6.xrenovate-bot/java-spanner-jdbc:renovate/com.google.cloud-google-cloud-spanner-bom-6.xCopy head branch name to clipboard