Create a model of the Spring Boot version to Gradle version mappings

[wilkinsona]

The versions of Gradle that different versions of Boot support and require are really hidden at the moment. They're expressed a little bit in JavaVersionProjectDescriptionCustomizer and also implicitly in the versions of Gradle that are bundled in the Initializr library. We need to express the mappings between versions more formally. This will hopefully open up some options for tackling things like spring-io/initializr#1162 and, as we figure out what to do, it may require some changes to the Initializr library.