Load git client from Antora instead of depending on it directly

To avoid having to maintain the dependency on git, load the one provided by the @antora/content-aggregator package. Note that this will require using at least Antora 3.1.2 since earlier versions of Antora had a dependency on isomorphic-git that's too old (we need at least 1.21.0).

We've gone back and forth about how to depend on the git client, but I'd prefer Collector to use whatever Antora provides and see how that works out.