Add details of upgrading to Gradle 5.0 (#6761) * Rename upgrading to Gradle 5.0 chapter
* Add Gradle 5.0 breaking changes to upgrade chapter
This commit groups and lists all the breaking changes that Gradle 5.0 introduces, many of which were earlier documented as deprecations.
* Try different headings in upgrade chapter
Trying out a change from "Changes in 4.x" to "Upgrading from 4.x and earlier". This is to see whether it's clearer to readers that they must apply all all sections up to and including the version they're upgrading from.