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/test/resources/org/springframework/boot/gradle/tasks/bundling/BootBuildImageIntegrationTests-failsWithBuilderError.gradle

@@ -7,5 +7,5 @@ sourceCompatibility = '1.8'
 targetCompatibility = '1.8'
 
  bootBuildImage {
-	 environment = ["BP_JAVA_VERSION" : "13.9.9"]
+	 environment = ["BP_JVM_VERSION" : "13.9.9"]
  }