Control the maven repos used by dependabot

Looking at the logs from when dependabot tries to update the dependency graph, it seems to be reading from the repository.jboss.org staging repo.

That's not the best place regardless, but the version update jobs are also failing due to timeout. So let's try explicitly declaring the repos in dependabot.yml and try and get the activity moved to maven central, or if not available the public repo in Nexus.