Always include plugin classpath in sample gradle

Update a couple of the `spring-boot-gradle-plugin` sample gradle flies
so that they include the running classpath. The additional lines are
contained within a tag which is ultimately filtered from the final
documentation.

Fixes gh-11857

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/getting-started.adoc

@@ -14,13 +14,13 @@ endif::[]
 ifeval::["{version-type}" == "MILESTONE"]
 [source,groovy,indent=0,subs="verbatim,attributes"]
 ----
-include::../gradle/getting-started/apply-plugin-milestone.gradle[]
+include::../gradle/getting-started/apply-plugin-milestone.gradle[tags=!buildclasspath]
 ----
 endif::[]
 ifeval::["{version-type}" == "SNAPSHOT"]
 [source,groovy,indent=0,subs="verbatim,attributes"]
 ----
-include::../gradle/getting-started/apply-plugin-snapshot.gradle[]
+include::../gradle/getting-started/apply-plugin-snapshot.gradle[tags=!buildclasspath]
 ----
 endif::[]
 
@@ -40,4 +40,4 @@ include::../gradle/getting-started/typical-plugins.gradle[tags=apply]
 ----
 
 To learn more about how the Spring Boot plugin behaves when other plugins are applied
-please see the section on <<reacting-to-other-plugins, reacting to other plugins>>.
\ No newline at end of file
+please see the section on <<reacting-to-other-plugins, reacting to other plugins>>.

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/getting-started/apply-plugin-milestone.gradle

@@ -5,6 +5,9 @@ buildscript {
 
 	dependencies {
 		classpath 'org.springframework.boot:spring-boot-gradle-plugin:{version}'
+// tag::buildclasspath[]
+		classpath files(pluginClasspath.split(','))
+// end::buildclasspath[]
 	}
 }
 

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/gradle/getting-started/apply-plugin-snapshot.gradle

@@ -5,6 +5,9 @@ buildscript {
 
 	dependencies {
 		classpath 'org.springframework.boot:spring-boot-gradle-plugin:{version}'
+// tag::buildclasspath[]
+		classpath files(pluginClasspath.split(','))
+// end::buildclasspath[]
 	}
 }