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

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 2.7.5 to 3.1.5.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.7.5...v3.1.5)

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

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

backend/api/build.gradle

@@ -23,7 +23,7 @@
  */
 
 plugins {
-    id 'org.springframework.boot' version '2.7.5'
+    id 'org.springframework.boot' version '3.1.5'
     id 'io.spring.dependency-management' version '1.1.3'
     id "org.jetbrains.kotlin.plugin.spring"
     id "org.jetbrains.kotlin.jvm"