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-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -182,7 +182,7 @@ the `spring-boot` plugin:
 			classpath("org.springframework.boot:spring-boot-gradle-plugin:{spring-boot-version}")
 		}
 	}
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot'
 ----
 
 If you are using a milestone or snapshot release you will also need to add appropriate

spring-boot-docs/src/main/asciidoc/deployment.adoc

@@ -387,7 +387,7 @@ With Gradle, the equivalent configuration would be:
 
 [source,groovy,indent=0,subs="verbatim,quotes,attributes"]
 ----
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot'
 
 	springBoot {
 		executable = true

spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -240,7 +240,7 @@ endif::release[]
 	}
 
 	apply plugin: 'java'
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot'
 
 	jar {
 		baseName = 'myproject'

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -257,7 +257,7 @@ endif::[]
 	}
 
 	apply plugin: 'java'
-	apply plugin: 'spring-boot'
+	apply plugin: 'org.springframework.boot'
 
 	repositories {
 ifeval::["{spring-boot-repo}" != "release"]