[Tech] bump org.jlleitschuh.gradle.ktlint in /backend

Bumps org.jlleitschuh.gradle.ktlint from 11.6.1 to 12.1.1.

---
updated-dependencies:
- dependency-name: org.jlleitschuh.gradle.ktlint
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

backend/build.gradle.kts

@@ -7,7 +7,7 @@ plugins {
     id("org.jetbrains.kotlin.plugin.allopen") version "2.0.0"
     kotlin("plugin.noarg") version "1.9.10"
     kotlin("plugin.jpa") version "2.0.0"
-    id("org.jlleitschuh.gradle.ktlint") version "11.6.1"
+    id("org.jlleitschuh.gradle.ktlint") version "12.1.1"
     kotlin("plugin.serialization") version "1.9.21"
 }