Bump org.mockito:mockito-core from 5.17.0 to 5.21.0 in /server

Bumps [org.mockito:mockito-core](https://github.com/mockito/mockito) from 5.17.0 to 5.21.0.
- [Release notes](https://github.com/mockito/mockito/releases)
- [Commits](mockito/mockito@v5.17.0...v5.21.0)

---
updated-dependencies:
- dependency-name: org.mockito:mockito-core
  dependency-version: 5.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

server/build.gradle

@@ -50,7 +50,7 @@ dependencies {
   testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
 
   // Mockito for testing
-  testImplementation 'org.mockito:mockito-core:5.17.0'
+  testImplementation 'org.mockito:mockito-core:5.21.0'
 }
 
 application {