SpringBoot Adjust to main branch in docs

Hi,

this PR adjust two more places where the new main branch naming of Spring projects could be used. It doesn't break anything because GitHub redirects properly, but it shows a banner at the top which is slightly annoying (at least for me).

Cheers, Christoph

Comment From: wilkinsona

Thanks very much, @dreis2211. I've taken care of the change to the dependency management plugin's docs location as part of some changes that I wanted to make across all three maintained branches.