SpringBoot Document that Java 15 is a supported version

Hi,

when working on JDK 15 support, I apparently forgot to adjust the system requirements section in the "Getting started" docs. This PR adjusts that.

Cheers, Christoph

Comment From: wilkinsona

Thanks, @dreis2211. I think we should wait until Java 15 reaches GA before documenting support for it. The mention in the release notes is sufficient for the time being.

Comment From: dreis2211

Fair enough