This commit improves the `InfoContributor` infrastructure as follows:

* `InfoEndpoint` no longer breaks its public API and returns a Map as
before
* `Info` is now immutable
* All properties of the build are now displayed. Since we control the
generation of that file, there is no longer a mode to restrict what's
shown
* Build info is now generated in `META-INF/build-info.properties` by
default

Closes gh-5734