SpringBoot Remove last mention of Gradle 5.6.x support from docs

Hi,

the getting started section still mentions Gradle 5.6 support, but that was dropped a short while ago.

Cheers, Christoph

Comment From: wilkinsona

Thanks, @dreis2211. I missed that one in https://github.com/spring-projects/spring-boot/issues/24225.

Comment From: wilkinsona

Thanks again, @dreis2211.