Automatically create a developmentOnly configuration for use with DevTools

[wilkinsona]

Following on from #14451, we could update the Gradle plugin to automatically create the developmentOnly configuration. This would remove the need for a user to define the custom configuration and adding DevTools in the recommended manner would become a one-line addition to the dependencies block:

developmentOnly 'org.springframework.boot:spring-boot-devtools'

Automatically creating the configuration would also mean that we'd know its name. This would hopefully allow us to exclude its dependencies from the classpath of the default BootJar and BootWar tasks, allowing us to deprecate the excludeDevtools property.