Update CNB buildpack Java version env variable

Prior to this commit, the build tool plugins set the environment
variable BP_JAVA_VERSION when invoking the CNB builder to set the
version of the JDK/JRE that the builder should use in the created
image.

With CNB API 0.3, the convention changed the name of this environment
variable to BP_JVM_VERSION. This commit updates the build tool
plugins to match the newer convention.

See gh-21273

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/asciidoc/packaging-oci-image.adoc

@@ -102,7 +102,7 @@ This configuration will use a builder image with the name `mine/java-cnb-builder
 ==== Builder Configuration
 If the builder exposes configuration options, those can be set using the `environment` property.
 
-The following example assumes that the default builder defines a `BP_JAVA_VERSION` property (typically used to customize the JDK version the image should use):
+The following example assumes that the default builder defines a `BP_JVM_VERSION` property (typically used to customize the JDK version the image should use):
 
 [source,groovy,indent=0,subs="verbatim,attributes",role="primary"]
 .Groovy

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-env.gradle

@@ -9,6 +9,6 @@ bootJar {
 
 // tag::env[]
 bootBuildImage {
-	environment = ["BP_JAVA_VERSION" : "13.0.1"]
+	environment = ["BP_JVM_VERSION" : "13.0.1"]
 }
 // end::env[]

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/docs/gradle/packaging/boot-build-image-env.gradle.kts

@@ -11,6 +11,6 @@ tasks.getByName<BootJar>("bootJar") {
 
 // tag::env[]
 tasks.getByName<BootBuildImage>("bootBuildImage") {
-	environment = ["BP_JAVA_VERSION" : "13.0.1"]
+	environment = ["BP_JVM_VERSION" : "13.0.1"]
 }
 // end::env[]