[build] Fix repositories order and plugin repositories
This commit reorders repositories to prioritize getting artifacts from
central repositories rather than the repo.spring.io (which now only
contains artifacts owned by the Spring and Reactor teams, and can also
ask for authentication).
It also ensures `snapshot`/`milestone`/`release` names are used, as the
`libs-` prefixed repositories are reserved for publishing artifacts.
Finally, it ensure gradle plugins are resolved from the jcenter
repository over the repo.spring.io one.
