gradle: bump org.springframework.boot from 2.7.4 to 3.0.1 in /backend

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 2.7.4 to 3.0.1.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.7.4...v3.0.1)

---
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/build.gradle

@@ -1,5 +1,5 @@
 plugins {
-    id 'org.springframework.boot' version '2.7.4'
+    id 'org.springframework.boot' version '3.0.1'
     id 'io.spring.dependency-management' version '1.1.0'
     id 'com.github.johnrengelman.shadow' version '7.1.2'
     id "io.freefair.lombok" version "6.6.1"