[validation] add validations for new Spring Boot versions in general

[martinlippert]

Beyond the initial validations for the generations (support ranges), we need to add validations for the Spring Boot versions in general (including corresponding preferences):

• newer patch version
• newer minor version
• newer major version

Each of those validations should mention the exact new version that is available. This should only take release versions into account, no snapshot or milestone or RC versions (yet).

[martinlippert]

Done.