[Uptime] Support agent data streams by default in Uptime

As part of https://github.com/elastic/uptime/issues/235 adding agent support to Kibana for Uptime we'll need to update our default index settings for Uptime to no longer just point at `heartbeat-*` since the datastreams will use the `synthetics` prefix. I believe the correct default should be `heartbeat-${VERSION}-*` and `synthetics-*` with no version.