change Microsoft.Insights/scheduledQueryRules enabled property to boolean

The enabled property is defined as a string enum with values **"true"** and **"false"**, as [the documentation](https://docs.microsoft.com/en-us/azure/templates/microsoft.insights/2018-04-16/scheduledQueryRules) reflects.

In other places - like [Microsoft.Insights/metricAlerts](https://docs.microsoft.com/en-us/azure/templates/microsoft.insights/2018-03-01/metricalerts) - the same property is defined as boolean, accepting values **true** and **false**.

Please change the property to boolean to improve consistency.

Start by locating the Microsoft.Insights scheduledQueryRules specification and read the definition of its enabled property. Confirm the current string enum and compare it with the metricAlerts definition; done means the scheduledQueryRules property accepts boolean true and false consistently.

Written by the indexing model from the issue text.