BE: Chore: Bump Spring Boot to 3.5.5

[yeikel]

Includes fix for CVE-2025-48989

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

• Unit checks
• Integration checks
• 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

[yeikel]

@Haarolean If you can 🙏