More aggressive pruning during GC

While investigating performance issues with the GitLab repo (https://gitlab.com/gitlab-com/gl-infra/scalability/-/issues/359#note_343823604), it was discovered that our house keeping uses the GC default of pruning only the last two weeks of loose objects. On a highly active repo, this allows hundreds of thousands of loose objects to accumulate. In the case of the GitLab repo, it had almost one million loose objects. These loose objects consume excessive disk space and may contribute to other performance issues.

Proposal

Use a more aggressive prune option during GC.