Align default step for /render with Grafana

In Grafana's native Explore Profiles view, the default step is 15
seconds. This is only used when the data source doesn't supply a
configured min step:

https://github.com/grafana/grafana/blob/24d65fa32f6ba7af3dc2e0ffa115c64aa7c97184/pkg/tsdb/grafana-pyroscope-datasource/query.go#L71-L87

pkg/querier/timeline/calculator.go

@@ -8,7 +8,7 @@ import (
 
 var (
 	DefaultRes         int64 = 1500
-	DefaultMinInterval       = time.Second * 10
+	DefaultMinInterval       = time.Second * 15
 )
 
 // CalcPointInterval calculates the appropriate interval between each point (aka step)