Add a second, portal-compatible ID to the Gradle plugin

To be compatible with Gradle's plugin portal, plugins must have an
ID that uses a reverse domain name. This means that spring-boot is
not compatible.

This commit introduces a new ID, org.springframework.boot, and
deprecates the old ID.

Closes gh-6997

spring-boot-samples/spring-boot-sample-logback/build.gradle

@@ -18,7 +18,7 @@ buildscript {
 apply plugin: 'java'
 apply plugin: 'eclipse'
 apply plugin: 'idea'
-apply plugin: 'spring-boot'
+apply plugin: 'org.springframework.boot'
 
 jar {
 	baseName = 'spring-boot-sample-simple'