Update log4j dependencies to v 2.16.0 (#287)

This version provides additional protection against CVE-2021-44228:
- https://issues.apache.org/jira/browse/LOG4J2-3208
- https://issues.apache.org/jira/browse/LOG4J2-3211

Author: diegov

examples/rollbar-log4j2/build.gradle

@@ -12,5 +12,5 @@ application {
 dependencies {
     implementation project(":rollbar-log4j2")
 
-    implementation group: 'org.apache.logging.log4j', name: 'log4j-slf4j-impl', version: '2.11.0'
+    implementation group: 'org.apache.logging.log4j', name: 'log4j-slf4j-impl', version: '2.16.0'
 }

rollbar-log4j2/build.gradle

@@ -1,6 +1,6 @@
 dependencies {
     api project(':rollbar-java')
 
-    api group: 'org.apache.logging.log4j', name: 'log4j-core', version: '2.15.0'
-    annotationProcessor 'org.apache.logging.log4j:log4j-core:2.15.0'
-}
+    api group: 'org.apache.logging.log4j', name: 'log4j-core', version: '2.16.0'
+    annotationProcessor 'org.apache.logging.log4j:log4j-core:2.16.0'
+}