MavenRepository should explicitly set whether release and snapshots are enabled

[snicoll]

The default for releases.enabled and snapshots.enabled are both true (see https://stackoverflow.com/questions/61684312/in-a-maven-repository-entry-in-the-pom-are-snapshots-enabled-by-default).

Right now, our API is such that releases are enabled by default (with no way to turn that off) and snapshots are disabled by default. We could keep drifting from those defaults but we should at least make sure they're properly applied in generated poms.

[sdeleuze]

For the record, here is what we configured in Spring Native with the goal of preventing downloads of snapshot dependencies from releases repositories and of release dependencies from snapshots repositories.