BE: Chore: Bump Spring Boot to 3.5.3

[yeikel]

What changes did you make? (Give an overview)

• Upgraded Spring Boot to the latest version (3.5.3) to fetch the latest bug fixes and features as well as fixing CVE-2025-41234
• Removed several dependency overrides that are no longer necessary since they were upgraded by Spring
• Removed the explicit inclusion of Junit5 , as it is provided transitively by spring-boot-starter-test. Keeping it explicitly declared leads to version conflicts, which goes against the JUnit project's recommendations for Spring Boot.

How Has This Been Tested? (put an "x" (case-sensitive!) next to an item)

• Covered by existing automation

Checklist (put an "x" (case-sensitive!) next to all the items, otherwise the build will fail)

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. ENVIRONMENT VARIABLES)
• My changes generate no new warnings (e.g. Sonar is happy)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged

A picture of a cute animal (not mandatory but encouraged)

[Haarolean]

@yeikel thank you!