Move CI to Concourse

A number of teams have moved their builds to https://ci.spring.io/ and crafted/shared scripts to automate the release process. That part alone is interesting as it is vastly superior to what the Bamboo integration can do.

I suggest we harmonize here as well, moving our `master` branch to Concourse, potentially cherry-picking in the `5.1.x` branch depending on the outcome of that work.
