Change frequency in Monitor Management to minutes

[paulb-elastic]

Browser tests are scheduled by the K8 cron scheduler, which has the smallest granularity of minutes. Although lightweight checks are going to initially be run from a long running container, we don’t know how this will be implemented in the future (and discussions to date have agreed on 1 minute being the most frequent).

We should consolidate / simplify the UI to specify the frequency in minutes for all types of monitor (just for the Monitor Management UI, not the Fleet Integration UI).

ACs:

• In Monitor Management (that is only enabled for those in the Tech Preview, where it has been explicitly enabled), the frequency should be defined in minutes
  • For all types of monitor (browser and lightweight)
  • Mandatory to be a whole integer, larger than zero
• There are no changes to the configuration options in the Fleet Integration UI

We should consider still sending the frequency configuration to the service in whatever unit makes sense and in such a way that it allows us options in the future for changing this to be more frequent than every minute, without it being a breaking change.