Upgrade to Spring Boot 3 / Spring Security 6

[pierocampa]

Spring Security 7 is already on its way, while our driver still relies on version 5: migrating to 6 (-> Spring Boot 3) is then starting to be an option.

Here's some useful information:

• Spring Security: Migrating to 6
• Spring Boot 3.0: Release Notes

[pierocampa]

Consider also re-using existing JWT Bearer token management Spring implementations:
https://docs.spring.io/spring-security/reference/servlet/oauth2/resource-server/jwt.html#oauth2resourceserver-jwt-minimaldependencies