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-integration-tests/spring-boot-gradle-tests/src/test/resources/executable-jar/build.gradle

@@ -12,7 +12,7 @@ repositories {
 	mavenCentral()
 }
 
-apply plugin: 'spring-boot'
+apply plugin: 'org.springframework.boot'
 apply plugin: 'java'
 
 dependencies {