Make start.spring.io first class citizen in the releaser

[marcingrzejszczak]

We tend to forget about checking start.spring.io after we make a release. We either

• should have an automated check for the Initlizr to see if the current release is not breaking anything there

and / or

• we should print out steps to be done after the release was finished (including manual checks for start.spring.io)

cc @snicoll