Bump org.springframework.boot from 2.6.3 to 3.2.3 in /app-backend

Bumps [org.springframework.boot](https://github.com/spring-projects/spring-boot) from 2.6.3 to 3.2.3.
- [Release notes](https://github.com/spring-projects/spring-boot/releases)
- [Commits](spring-projects/spring-boot@v2.6.3...v3.2.3)

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

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

Author: dependabot[bot]

app-backend/build.gradle

@@ -1,5 +1,5 @@
 plugins {
-	id 'org.springframework.boot' version '2.6.3'
+	id 'org.springframework.boot' version '3.2.3'
 	id 'io.spring.dependency-management' version '1.1.4'
 	id 'java'
 }