build(deps): bump org.springframework:spring-test in /backend

Bumps [org.springframework:spring-test](https://github.com/spring-projects/spring-framework) from 5.3.10 to 6.1.6.
- [Release notes](https://github.com/spring-projects/spring-framework/releases)
- [Commits](spring-projects/spring-framework@v5.3.10...v6.1.6)

---
updated-dependencies:
- dependency-name: org.springframework:spring-test
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/build.gradle

@@ -77,7 +77,7 @@ dependencies {
   testImplementation 'org.mockito:mockito-all:1.10.19'
   testImplementation 'org.springframework:spring-core:5.3.10'
   testImplementation 'org.springframework:spring-web:5.3.10'
-  testImplementation 'org.springframework:spring-test:5.3.10'
+  testImplementation 'org.springframework:spring-test:6.1.6'
 
   testImplementation 'com.google.appengine:appengine-testing:+'
   testImplementation 'com.google.appengine:appengine-api-stubs:+'