[Tech] Bump org.jetbrains.kotlin:kotlin-stdlib-jdk8 in /backend

Bumps [org.jetbrains.kotlin:kotlin-stdlib-jdk8](https://github.com/JetBrains/kotlin) from 1.9.10 to 2.0.20.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.10...v2.0.20)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib-jdk8
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/build.gradle.kts

@@ -60,7 +60,7 @@ dependencies {
     api("org.flywaydb:flyway-database-postgresql:10.15.2")
     api("org.springdoc:springdoc-openapi-ui:1.8.0")
     api("org.jetbrains.kotlin:kotlin-reflect:1.9.23")
-    api("org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.9.10")
+    api("org.jetbrains.kotlin:kotlin-stdlib-jdk8:2.0.20")
     api("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.8.1")
     api("com.neovisionaries:nv-i18n:1.29")
     api("com.github.ben-manes.caffeine:caffeine:3.1.8")