bump log4j version to 2.15.0 (#13494)

Author: jsvd

logstash-core/build.gradle

@@ -30,7 +30,7 @@ String jrubyVersion = versionMap['jruby']['version']
 String jacksonVersion = versionMap['jackson']
 String jacksonDatabindVersion = versionMap['jackson-databind']
 
-String log4jVersion = '2.14.0'
+String log4jVersion = '2.15.0'
 
 repositories {
  mavenCentral()