Upgrade Log4j to 2.15.0 (Tests Only)

There is no runtime dependency on Log4j.

Author: garyrussell

build.gradle

@@ -59,7 +59,7 @@ ext {
 	junit4Version = '4.13.1'
 	junitJupiterVersion = '5.7.2'
 	kafkaVersion = '2.6.3'
-	log4jVersion = '2.13.3'
+	log4jVersion = '2.15.0'
 	micrometerVersion = '1.5.17'
 	mockitoVersion = '3.6.28'
 	reactorVersion = 'Dysprosium-SR25'