[Tech] Bump io.ktor:ktor-client-mock-jvm from 2.3.3 to 2.3.9 in /backend

Bumps [io.ktor:ktor-client-mock-jvm](https://github.com/ktorio/ktor) from 2.3.3 to 2.3.9.
- [Release notes](https://github.com/ktorio/ktor/releases)
- [Changelog](https://github.com/ktorio/ktor/blob/2.3.9/CHANGELOG.md)
- [Commits](ktorio/ktor@2.3.3...2.3.9)

---
updated-dependencies:
- dependency-name: io.ktor:ktor-client-mock-jvm
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/build.gradle.kts

@@ -70,7 +70,7 @@ dependencies {
     api("io.sentry:sentry:6.31.0")
     api("io.sentry:sentry-log4j2:7.5.0")
     runtimeOnly("org.postgresql:postgresql:42.6.0")
-    testImplementation("io.ktor:ktor-client-mock-jvm:2.3.3")
+    testImplementation("io.ktor:ktor-client-mock-jvm:2.3.9")
     testImplementation("org.assertj:assertj-core:3.25.0")
     testImplementation("org.testcontainers:postgresql:1.19.4")
     testImplementation("org.testcontainers:testcontainers:1.19.4")