SpringBoot Note the Kt class name suffix when describing how to configure a Kotlin application's main class name

The purpose of this PR is to make it clearer that when Kotlin is used, the name of the main class is not equal to the name of the Kotlin class. This is unrelated with what language the Gradle build script is written in.

In issue gh-22664 this confusion was brought up.

Comment From: wilkinsona

Thanks very much, @fernandezseb. I've merged this into 2.2.x and then forwards so it'll be part of the next 2.3.x and 2.4.x releases as well.

Comment From: fernandezseb

@wilkinsona Thanks for your support!