Upgrade to Spring Boot 4.1.0-M2

[sdeleuze]

As discussed with the team:

• Spring AI 2.0 should be built against Spring Boot 4.1
• A CI job to run the tests against Spring Boot 4.0 should be added
• The documentations (README, reference) should clame a Spring Boot 4.0 / Spring Framework 7.0 baseline

[ilayaperumalg]

Updated to Spring Boot 4.1.0-M2 via ce91f4c. Preliminary work on this was done via 81188c9