Allow purging available rollbacks before their TTL has expired #10695
Description
Allow purging available rollbacks that are stored on disk before their TTL has expired for both standalone and Fleet managed agents.
In the current implementation the TTL for a version gets set at the time of the upgrade action introducing the rollback version with no way to accelerate or immediately delete it from disk. In the theme of handling the unexpected, if a user needs to free up disk space taken up by a rollback version, there is no way for that to happen besides waiting and we should add an alternative.
The simplest mechanism for this could be that setting agent.upgrade.rollback.window: 0 to disable the rollback feature also purges all locally stored rollback versions. This would work for standalone and fleet managed agents without needing to introduce any additional fleet action since it is controlled via a policy change.