Fix deprecation warning:

Replace the mistakenly imported assertThat method with the main one.

Author: dmitry-timofeev

exonum-java-binding/core/src/test/java/com/exonum/binding/core/storage/database/NativeResourceManagerIntegrationTest.java

@@ -16,7 +16,7 @@
 
 package com.exonum.binding.core.storage.database;
 
-import static org.assertj.core.api.Java6Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import com.exonum.binding.common.serialization.StandardSerializers;