Put back "severity" and "riskScore" in the form schema

The way the information about a rule is shown depends on keys defined on the form schema. This is a potential future bug as there is only 1 field declared in the JSX (e.g. "riskScore") so what is inside the object is expected to be FieldConfig properties.

x-pack/plugins/security_solution/public/detections/components/rules/step_about_rule/schema.tsx

@@ -101,6 +101,16 @@ export const schema: FormSchema = {
     ),
     labelAppend: OptionalFieldLabel,
   },
+  severity: {
+    value: {},
+    mapping: {},
+    isMappingChecked: {},
+  },
+  riskScore: {
+    value: {},
+    mapping: {},
+    isMappingChecked: {},
+  },
   references: {
     label: i18n.translate(
       'xpack.securitySolution.detectionEngine.createRule.stepAboutRule.fieldReferenceUrlsLabel',