Upgrade to Spring Boot 2.3
Upgrade to Spring Boot 2.3

This commit completely refactors the Sagan Site build, by upgrading the
application to Spring Boot 2.3. We're making here the required
structural changes, like applying the dependency management plugin.

We're also removing overridden dependency versions since the latest
dependency management provides up-to-date versions.

spring-io/sagan,spring-io/sagan,spring-io/sagan,spring-io/sagan