Bump mongo-java-driver from 3.12.3 to 3.12.4 in /core (#2676)

Author: dependabot-preview[bot]

core/build.gradle

@@ -157,7 +157,7 @@ dependencies {
     testCompile 'org.apache.httpcomponents:httpclient:4.5.9'
     testCompile 'redis.clients:jedis:3.3.0'
     testCompile 'com.rabbitmq:amqp-client:5.9.0'
-    testCompile 'org.mongodb:mongo-java-driver:3.12.3'
+    testCompile 'org.mongodb:mongo-java-driver:3.12.4'
 
     testCompile ('org.mockito:mockito-core:3.3.3') {
         exclude(module: 'hamcrest-core')