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

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 3.4.4 to 3.5.5.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v3.4.4...v3.5.5)

---
updated-dependencies:
- dependency-name: org.springframework.boot
  dependency-version: 3.5.5
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/build.gradle

@@ -1,6 +1,6 @@
 plugins {
 	id 'java'
-	id 'org.springframework.boot' version '3.4.4'
+	id 'org.springframework.boot' version '3.5.5'
 	id 'io.spring.dependency-management' version '1.1.7'
 	id 'com.diffplug.spotless' version '7.2.1'
 }