Update .fleet-agent index mappings in elasticsearch for rollback fields

Requires https://github.com/elastic/fleet-server/pull/5975

After the fleet-server new fields for `.fleet-agent` index are finalized, add the correct mapping in elasticsearch (similar to [what's been done for rollback `reason`](https://github.com/elastic/elasticsearch/pull/134711) minus the backports).

This will ensure that the new fields of `available_rollbacks` are properly mapped and indexed