Upgrade Log4j to 2.15.0 (Tests Only)

There is no runtime dependency on Log4j.

Author: garyrussell

build.gradle

@@ -60,7 +60,7 @@ ext {
 	junit4Version = '4.13.2'
 	junitJupiterVersion = '5.7.2'
 	kafkaVersion = '2.7.2'
-	log4jVersion = '2.14.1'
+	log4jVersion = '2.15.0'
 	micrometerVersion = '1.6.13'
 	mockitoVersion = '3.9.0'
 	reactorVersion = '2020.0.13'