Review instructions for running launch script tests with Gradle

Closes gh-20606

Author: snicoll

spring-boot-tests/spring-boot-integration-tests/spring-boot-launch-script-tests/README.adoc

@@ -38,11 +38,14 @@ user to be able to connect to the daemon.
 
 == Running the tests
 
+NOTE: You need Java 8 to run the integration tests as they are currently skipped for Java
+9 and later.
+
 You're now ready to run the tests. Assuming that you're in the same directory as this
 README, the tests can be launched as follows:
 
 ----
-$ mvn -Pdocker clean verify
+$ gradle intTest
 ----
 
 The first time the tests are run, Docker will create the container images that are used to