build(deps): bump org.springframework.boot in /backend

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 3.4.4 to 3.5.4.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v3.4.4...v3.5.4)

---
updated-dependencies:
- dependency-name: org.springframework.boot
  dependency-version: 3.5.4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/build.gradle.kts

@@ -2,7 +2,7 @@ plugins {
     kotlin("jvm") version "2.1.20"
     kotlin("plugin.spring") version "2.1.20"
 
-    id("org.springframework.boot") version "3.4.4"
+    id("org.springframework.boot") version "3.5.4"
     id("io.spring.dependency-management") version "1.1.7"
 
     id("com.netflix.dgs.codegen") version "7.0.3"