Updated the Java main class package name to org.opensearch.

Signed-off-by: David Venable <dlv@amazon.com>

data-prepper-core/build.gradle

@@ -51,7 +51,7 @@ jar {
     manifest {
         attributes('Implementation-Title': project.name,
                 'Implementation-Version': project.version,
-                'Main-Class': 'com.amazon.dataprepper.DataPrepperExecute')
+                'Main-Class': 'org.opensearch.dataprepper.DataPrepperExecute')
     }
     from {
         configurations.runtimeClasspath.collect { it.isDirectory() ? it : zipTree(it) }