build(deps-dev): bump testng from 6.8 to 7.7.0 in /backend

Bumps [testng](https://github.com/cbeust/testng) from 6.8 to 7.7.0.
- [Release notes](https://github.com/cbeust/testng/releases)
- [Changelog](https://github.com/cbeust/testng/blob/master/CHANGES.txt)
- [Commits](https://github.com/cbeust/testng/compare/testng-6.8...7.7.0)

---
updated-dependencies:
- dependency-name: org.testng:testng
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/pom.xml

@@ -185,7 +185,7 @@
         <dependency>
             <groupId>org.testng</groupId>
             <artifactId>testng</artifactId>
-            <version>6.8</version>
+            <version>7.7.0</version>
             <scope>test</scope>
         </dependency>
         <!-- ehcache 缓存 -->