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

[pchila]

Requires #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 minus the backports).

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

[ebeahan]

PR to implement: elastic/elasticsearch#139363