Alerting: Change Data to use Labels instead of map[string]string (#63431)

This commit changes the Data struct in template.go to use Labels
instead of map[string]string. It changes how labels are printed
when using {{ .Labels }} from map[foo:bar bar:baz] to
foo=bar, bar=baz.

pkg/services/ngalert/state/template/template.go

@@ -70,7 +70,7 @@ func NewValues(caps map[string]eval.NumberValueCapture) map[string]Value {
 }
 
 type Data struct {
-	Labels map[string]string
+	Labels Labels
 	Values map[string]Value
 	Value  string
 }