deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.20.0 (#3713)
deps: update dependency com.google.cloud:google-cloud-spanner-bom to v6.20.0 (#3713)


googleapis/java-cloud-bom,googleapis/java-cloud-bom,googleapis/java-cloud-bom,googleapis/java-cloud-bom