chore(deps): bump com.diffplug.spotless in /backend

Bumps com.diffplug.spotless from 6.13.0 to 6.25.0.

---
updated-dependencies:
- dependency-name: com.diffplug.spotless
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

backend/build.gradle

@@ -23,7 +23,7 @@
  */
 plugins {
     id "pl.allegro.tech.build.axion-release" version "1.18.8"
-    id "com.diffplug.spotless" version "6.13.0"
+    id "com.diffplug.spotless" version "6.25.0"
     id "org.jetbrains.kotlin.jvm" version "2.0.20"
     id "org.jetbrains.kotlin.plugin.allopen" version "2.0.20"
 }