fix: tests/milvus-java-test/pom.xml to reduce vulnerabilities (#3663)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JAVA-COMBEUST-174815

tests/milvus-java-test/pom.xml

@@ -91,7 +91,7 @@
         <dependency>
             <groupId>org.testng</groupId>
             <artifactId>testng</artifactId>
-            <version>6.14.3</version>
+            <version>7.3.0</version>
         </dependency>
 
         <dependency>