[Tech] Bump org.testcontainers:postgresql in /backend

Bumps [org.testcontainers:postgresql](https://github.com/testcontainers/testcontainers-java) from 1.19.4 to 1.19.6.
- [Release notes](https://github.com/testcontainers/testcontainers-java/releases)
- [Changelog](https://github.com/testcontainers/testcontainers-java/blob/main/CHANGELOG.md)
- [Commits](testcontainers/testcontainers-java@1.19.4...1.19.6)

---
updated-dependencies:
- dependency-name: org.testcontainers:postgresql
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

backend/build.gradle.kts

@@ -72,7 +72,7 @@ dependencies {
     runtimeOnly("org.postgresql:postgresql:42.6.0")
     testImplementation("io.ktor:ktor-client-mock-jvm:2.3.3")
     testImplementation("org.assertj:assertj-core:3.25.0")
-    testImplementation("org.testcontainers:postgresql:1.19.4")
+    testImplementation("org.testcontainers:postgresql:1.19.6")
     testImplementation("org.testcontainers:testcontainers:1.19.4")
     testImplementation("org.testcontainers:junit-jupiter:1.19.4")
     testImplementation("jakarta.servlet:jakarta.servlet-api:6.0.0")