SpringBoot Document the need to use the launcher to be able to use application.* properties in a custom banner

This is a documentation update to describe the need to use JarLauncher to use application.* properties in custom banner. This is in response to issue 22519

Comment From: philwebb

Thanks @robert-smith-911. I've merged this to 2.3.x and forward with a slight tweak to the wording since you you don't need the -cp . option when using the JarLauncher.

Comment From: robert-smith-911

Thanks @philwebb !