Bump testng from 7.6.1 to 7.8.0 in /dockerfile-image-update

Bumps [testng](https://github.com/testng-team/testng) from 7.6.1 to 7.8.0.
- [Release notes](https://github.com/testng-team/testng/releases)
- [Changelog](https://github.com/testng-team/testng/blob/master/CHANGES.txt)
- [Commits](testng-team/testng@7.6.1...7.8.0)

---
updated-dependencies:
- dependency-name: org.testng:testng
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

dockerfile-image-update/pom.xml

@@ -87,7 +87,7 @@
         <dependency>
             <groupId>org.testng</groupId>
             <artifactId>testng</artifactId>
-            <version>7.6.1</version>
+            <version>7.8.0</version>
             <scope>test</scope>
         </dependency>
         <dependency>