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

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 3.1.0 to 4.0.2.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v3.1.0...v4.0.2)

---
updated-dependencies:
- dependency-name: org.springframework.boot
  dependency-version: 4.0.2
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

Author: dependabot[bot]

backend/build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
 	java
-	id("org.springframework.boot") version "3.1.0"
+	id("org.springframework.boot") version "4.0.2"
 	id("io.spring.dependency-management") version "1.1.0"
 }