Add log4j as Slf4j Impl.

Since SLF4J is only a logging facade and does not
provide a concrete logging implementation, we need
to include `log4j-slf4j-impl` as a dependency.
This allows SLF4J to route logging calls to Log4j-2,
which acts as the backend logging framework.

java/app/build.gradle

@@ -62,6 +62,8 @@ dependencies {
 
   compileOnly 'org.projectlombok:lombok:1.18.24'
   annotationProcessor 'org.projectlombok:lombok:1.18.24'
+  implementation "org.slf4j:slf4j-api:1.7.36"
+  runtimeOnly 'org.apache.logging.log4j:log4j-slf4j-impl:2.24.2'
 
   testImplementation "org.junit.jupiter:junit-jupiter-api:$junitVersion"
   testImplementation "org.junit.jupiter:junit-jupiter-params:$junitVersion"