bump log4j from 2.11.0 to 2.15.0 in gradle.properties (#1021)
bump log4j from 2.11.0 to 2.15.0 in gradle.properties (#1021)

see https://github.com/vorburger/Log4j_CVE-2021-44228/
google/data-transfer-project,google/data-transfer-project,google/data-transfer-project,google/data-transfer-project,google/data-transfer-project